# Rocks and Minerals (A-E)

Acidic rock
Acidic rock refers to an igneous rock that consists mostly of light coloured minerals and has more than 66% free or combined silica.

Actinolite
Actinolite has the formulae Ca2(Mg,Fe)5Si8O22(OH)2 and a relative hardness of 6.
It commonly occurs in crystalline schists, often being the chief constituent of green-coloured schists and greenstones. It is used somewhat as an asbestos material.

Adamantine
Adamantine refers to a brilliant luster like that of a diamond. Results from a mineral having a high index of refraction.

Adamite
Adamite has the formulae Zn2(AsO4)(OH) and a relative hardness of 4.
It has brilliant fluorescence, is of scientific interest and is much desired by collectors.

Adularia
Adularia (Moonstone) is a semi-precious, clear, transparent, glassy form of potash felspar which is found mainly in the crevices of crystalline schists and gneisses, often in beutifully perfect crystals. It has sometimes a pearly, opalescent reflection or play of colours.

Aegirite
Aegirite has the formulae NaFe(Si2O6) and a relative hardness of 7.
It is a rock-forming mineral found mainly in rocks rich in soda and poor in silica. Named after Aegir, Icelandic god of the sea.

Agalmatolite
Agalmatolite is a soft species of mineral, also called pagodite and figure-stone, used by the Chinese for carving, especially into grotesque figures.

Agaric Mineral
Agaric Mineral is one of the purest native carbonates of lime. It is chiefly found in the clefts of rocks and at the bottom of some lakes.

Agate
Agate is a semi-precious stone comprised mainly of silica. It is a compact variety of chalcedony. It has a pattern of banding which occurs due to chemical admixtures of haematite, limonite and other minerals.

Alabaster
Alabaster is naturally occurring hydrated calcium sulphate.

Albite
Albite has the formulae NaAlSi3O8 and a relative hardness of 7.
It is widely distributed, rock-forming mineral. Present in pegmatite dikes and may be found in crystals.

Alkaline rock
Alkaline rock refers to any rock which contains more than average amounts of potassium bearing and sodium bearing minerals.

Allanite
Allanite has the formulae (Ce,Ca,Y)2(Al,Fe)3(SiO4)3(OH) and a relative hardness of 6.
It occurs as a minor constituent of many igneous rocks and is frequently associated with epidote. Found in some magnetic bodies.

Allemontite
Allemontite is a natural alloy of arsenic and antimony. It has a relative hardness of 4.

Almandine
Almandine is a variety of precious garnet, reddish or violet in colour.

Alteration
Alteration refers to physical or chemical change in a rock or mineral after its original formation. Can result in new minerals or in textural changes in the rock.

Alumina
Alumina is aluminium oxide and occurs as ruby, sapphire, bauxite.

Alunite
Alunite has the formulae KAl3(SO4)2(OH)6 and a relative hardness of 4.
It is usually formed by sulphuric acid solutions acting on rocks rich in alkalic feldspar. Used in the production of alum.

Alunogen
Alunogen is a fibrous aluminium sulphate found in volcanic debris, clays, feldspathic rocks which contain pyrites, and often as an inflorescence on the walls of mines and caves.

Amblygonite
Amblygonite has the formulae (Li,Na)Al(PO4)(F,OH).
It has a relative hardness of 6.
It occurs in lithium and phosphate rich granite pegmatites. Associated with spodumene, apatite, tourmaline, lepidolite.

Amethyst
Amethyst is a precious stone. It is quartz coloured with manganese.

Amorphous
Amorphous refers to 'Without form'. The term is applied to rocks and minerals that lack definite crystal structure.

Amphibole
The amphiboles are a large group of minerals, the silicates of many different bases, the commonest being alumina, iron oxide, lime, magnesia and the alkalis. They are constituents of many crystalline igneous rocks and of metamorphic schists. In many of their properties they closely resemble the pyroxenes. They occur generally in black or dark green crystals, usually long, narrow and blade-like, and owing to their perfect cleavages their surfaces are smooth and bright. The commoner varieties are hornblende, actinolite and tremolite.

Amygdaloidal
Amygdaloidal is a name given to igneous rocks, usually old lava flows, full of almond-shaped cavities which have been filled up with secondary minerals, such as calcite, agate or the zeolites. These cavaties vary in size up to several centimeters across and were formed while the rock was still fluid and in motion.

Amygdule
Amygdule refers to a mineral containing cavity in an igneous rock formed by escaping gas.

Anaclime
Anaclime has the formulae NaAlSi2O6ù2H2O and a relative hardness of 6.
It is found in the cavities of intrusive and volcanic igneous rocks; often as clear shiny crystals and is associated with calcite and zeolites.

Anatase
Anatase has the formulae TiO2 and a relative hardness of 6.
It is a form of rutile found in granite, gneiss, mica schist, maetamorphic limestone, and dolomite. May be present as an accessory mineral in the rocks or in a quartz vein traversing it. It is used as a coating for welding rods and as a derivative of titanium.

Andalusite
Andalusite is a mineral formed by the metamorphism of aluminous shales and slate. It is used in the manufacture of spark plugs and other porcelains and may serve as a gem stone if it is clear and transparent. It is named from Andalusia, a province of Spain. It has the formulae Al2SiO5 and a relative hardness of 8.

Andesine
Andesine has the formulae NaAlSi3O8.
It has a relative hardness of 6.

Andesite
Andesite is a crystalline igneous rock, occuring mostly in lava flows, but sometimes in dykes and veins. It consists proncipally of plagioclase felspar, and is often porphyritic, showing large crystals of felspar scattered through a fine-grained mass, usually of small felspar crystals, but often containing much glassy material. Andesite forms most of the recent volcanic rock of the Andes, and is thus named after the Andes.

Anglesite
Anglesite is a common, minor ore of lead formed by the oxidation of galena.It has the formulae PbSO4 and a relative hardness of 3. It is found in the upper, oxidzed portions of lead veins and is named after the Island of Anglesey.

Anhydrite
Anhydrite is a mineral consisting of sulphate of lime. It has rectangular cleavage fragments and occurs in a manner similar to gypsum and often found with it but not as common. It is found in the cap rock of salt domes and in limestone rocks. The name comes from the Greek meaning 'without water'. It has the formulae CaSO4 and a relative hardness of 4.

Annivite
Annivite is a variety of terahedrite containing bismuth and usually iron and zinc.

Anorthite
Anorthite is a widely distributed and abundant rock-forming feldspar. It has the formulae (Na,Ca)AlSi3O8 and a relative hardness of 7.

Anthophyllite
Anthophyllite is a mineral occuring in crystalline schists rich in magnesium. It has the formulae (Mg,Fe)7Si8O22(OH)2 and a relative hardness of 6.

Anthracite
Anthracite (stone coal) is a kind of coal distinguished by its great hardness, its high proportion of carbon, and the great heat given out in burning.

Antimonite
Antimonite is an antimony ore. It was used by the Greeks as a cosmetic for darkening the eyelids.

Antimony
Antimony has the formulae Sb.
It has a relative hardness of 4.
It is a native semimetal and difficult to distinguish from arsenic. Rather brittle and a poorer conductor of heat and electriciy than native metals.

Apatite
Apatite has the formulae Ca5(F,Cl,OH)(PO4)3.
It has a relative hardness of 5.
It is widely distributed as an accesory constituent in all classes of rock. Found in crystals and masses. Crystallized apatite has been extensivley for fertilizer. Transparent varieties of fine colour are sometimes used for gems - but it's too soft for extensive use.

Aphanitic rock
Aphanitic rock refers to a rock in which the crystalline constituents are too small to be distinguished without magnification.

Apophyllite
Apophyllite has the formulae KCa4Si8O2O(OH)ù8H2O.
It has a relative hardness of 5.
It is a hydrothermal mineral filling cavities in basalt and tufaceous rocks. Associated with stilbite, scolecite, calcite, prehnite, analcime.

Aragonite
Aragonite has the formulae CaCO3.
It has a relative hardness of 4.
It forms easily in a sedimentary environment but only stable in metamorphic rocks formed at high temperatures.

Arfvedsonite
Arfvedsonite has the formulae Na2-3(Fe,Mg,Al)5Si8O22(OH,F)2.
It has a relative hardness of 6.
It is typical of alkaline plutonic rocks that are rich in iron.

Argentite
Argentite is an important primary silver ore. The name is from Latin meaning 'silver'. Occurs disseminated in galena and in the cementation zone of lead and zinc deposits. Associated with cerussite, chlorargyrite, native silver. It has the formulae Ag2S and a relative hardness of 3.

Argillaceous
Argillaceous refers to composed mostly of or containg clay; such as shale.

Arsenates
Arsenates refers to minerals in which arsenate (AsO4) is an important part.

Arsenic
Arsenic is a a trivalent and pentavalent, solid, poisonous element that is commonly metallic steel-gray, crystalline and brittle. It is used in medicine and in the manufacture of electronic components. It has a garlicky odor but otherwise it is difficult to distinguish from antimony. It is a relatively rare mineral found in veins in crystalline rocks associated with silver, cobalt, or nickel ores. It has the symbol As.

Arsenopyrite
Arsenopyrite is the most common mineral containing arsenic. It occurs with tin and tungsten ores in high temperature deposits as a deposition from hot waters. Frequently associated with gold. Also found with copper and silver ores. It has the formulae FeAsS and a relative hardness of 6.

Asbestos
Asbestos is a fibrous mineral with a high melting and combustion point. Cloth was made of it by the ancient Egyptians.

Atacamite
Atacamite is the native hydrous oxychloride of copper. It is a comparatively rare mineral occuring in arid regions in the upper oxidized zone of copper deposits. It has the formulae Cu2Cl(OH)3 and a relative hardness of 4.

Augite
Augite has the formulae (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6.
It has a relative hardness of 6.
It is an important rock forming mineral. Chiefly found in dark coloured igneous rocks, especially those whose magmas were rich in iron, calcium and magnesium. Seldom found in rocks that contain much quartz. A common memeber of the pyroxene group. Told from the amphibole group by cleavage.

Autunite
Autunite has the formulae Ca(UO2)2(PO4)2ù10-12H2O.
It has a relative hardness of 3.
It has strong fluorescence. A secondary mineral found in the zone of oxidation and weathering of uranite or other uranium minerals. Used as an ore of uranium.

Axinite
Axinite has the formulae Ca2(Fe,Mn)Al2(BO3)(Si4O12)(OH).
It has a relative hardness of 7.
It occurs in cavities in granite and in the contact zones surrounding granite intrusions.

Azurite
Azurite has the formulae Cu3(CO3)2(OH)2.
It has a relative hardness of 4.
It is a minor ore of copper with an intense azure-blue colour. Alters to malachite. Associated with limonite, calcite, chalcocite, chrysocolla and other secondary copper minerals. Reacts vigorously with hydrochloric acid.

Barite
Barite has the formulae BaSO4.
It has a relative hardness of 3.
It is major ore of barium. Has a high SG for a light coloured mineral. Common gangue mineral in hydrothermal veins or as a repacement mineral in veins of limestone and dolomite. Associated with lead, silver and antimony sulphides.

Barytes
Barytes was the first mineral to be found to be luminescent when heated, and led to the discovery of the luminescence of minerals. It has a relative hardness of 3.5.

Basanite
Basanite is a smooth, black siliceous mineral.

Basic rock
Basic rock refers to an igneous rock with a low percentage of silica and a high percentage of pyroxene, hornblende, and labradorite.

Bauxite
Bauxite is a sedimentary rock group of various aluminium oxides, a principal ore of aluminium, found in France and Jamaica. Bauxite was named after the place where it was first found, Les Baux in France. It has a relative hardness of 1 to 3.

Bedding
Bedding refers to the arrangement of sedimentary rocks in about parallel layers or strata which correspond to the original sediments.

Beryl
Beryl (beryllium aluminium meta-silicate) has the formulae Be3Al2Si16O18 and a relative hardness of 8.
It has a characteristic six-sided outline and is used as a gem stone of various colours, its green variety being emerald. Beryl is also the major source of the rare element beryllium, a light metal similar to aluminum. Beryl is quite common and occurs usually in granite rocks, mica schists and with tin ores.

Beryllium aluminium meta-silicate
see "Beryl"

Biotite
Biotite has the formulae K(Mg,Fe)2(Al,Fe)Si3O10(OH,F)2 and a relative hardness of 3.
It is a widely distributed rock forming mineral and occurs in igneous and metamorphic rocks and is a common member of the mica group.

Bismuthinite
Bismuthinite has the formulae Bi2S3 and a relative hardness of 2. It is a rare ore of bismuth. Occurs in veins that show definite relations to igneous rocks.

Bituminous rocks
Bituminous rocks refers to rocks that contain tar, petroleum, or asphalt.

Blende
Blende is a zinc ore. It generally contains more than half its weight in zinc, a quarter sulphur and often a small amount of iron.

Bloodstone
Bloodstone is a dark green variety of chalcedony.

Bohemian garnet
see "Pyrope"

Bole
Bole is an earthy mineral occuring in amorphous masses , and composed chiefly of silica with alumina, iron and occasionally magnesia.

Boracite
Boracite has the formulae Mg3B7O13Cl and a relative hardness of 7. It occurs associated with beds of halite, anhydrite, and gypsum. It is formed by the evaporation of bodies of salt water.

Borates
Borates refers to a group of minerals in which the borate radical (BO3) is an important constituent.

Borax
Borax (sodium tetraborate) is the sodium salt of pyroboric acid. It has the formulae Na2B4O710H2O and a relative hardness of 3. It forms large transparent six-sided prisms which have an alkaline reaction, effloresce in air, and when heated swell-up and melt to a transparent glass. Borax is used in the manufacture of enamel-ware, glass, as an antiseptic and is a food preservative. It is also useful in brazing and silver soldering as it dissolves metalic oxides, thus cleaning the surfaces of the metals to be united.

Bornite
Bornite has the formulae Cu5FeS4 and a relative hardness of 3. It is an ore of copper with a colourful tarnish, widely occurring, it is found in basic rocks and metamorphic deposits.

Botryoidal
Botryoidal refers to resembling a bunch of grapes. A mineral of this type appears to have a surface covered with spherical bulges.

Boulangerite
Boulangerite is a lead ore. It contains 55 percent lead. It has a relative hardness of 2.5.

Bournonite
Bournonite is an ore of lead, copper, and antimony which often exhibits twinned crystals. It occurs in veins formed at moderate temperatures and has the formulae PbCuSbS3 and a relative hardness of 3.

Breccia
Breccia is a rock consisting of angular fragments of any kind, united by a matrix. The shape of the components indicates that they have been produced by fracture, and have not been subjected to rounding by attrition. Fault breccia is often found between the two walls of a geological fault, and is due to the breaking down of the rocky walls when grinding on one another. Mineral veins are often formed in fissures, and are brecciated later by movement of the walls. Another kind of breccia is produced when hot molten lava enters a lake or a stream; it is suddenly cooled and solidified, being at the same time shattered by the clouds of steam that are formed. Braccias differ from conglomerates in the angular nature of their fragments, and in the method of their origin.

Brimstone
Brimstone is another name for sulphur.

Bronzite
Bronzite is a pyroxene which is co called from its sub-metallic lustre resembling tarnished bronze.

Brookite
Brookite is a source of titanium but deposits are usually too small to be of commercial use. It has the formulae TiO2 and a relative hardness of 6.

Brown Spar
Brown Spar is the name given to some crystalline varieties of dolomite tinged with peroxide of Iron

Brucite
Brucite is a decomposition product of magnesium silicates, especially serpentine. It has the formulae Mg(OH)2 and a relative hardness of 3.
It is found in Texas, where it is white with a grey, green or blue tinge and is used in sugar-refining.

Cacholong
Cacholong or mother-of-pear opal and sometimes Kalmuck agate, is a variety of opal, usually grey in colour, milk white or bluish white, and resembling mother-of-pearl. It is banded with layers of different colours and is a most attractive ornamental stone.

Calamine
Calamine is a zinc ore.

Calcareous
Calcareous refers to containing calcium carbonate or calcite.

Calcic
Calcic refers to containing calcium.

Calcite
Calcite has the formulae CaCO3.
It has a relative hardness of 3.
It effervesces vigorously with HCl. Clear specimens exhibit double refraction. Occurs as widespread sedimentary rock masses such as limestone. Crystalline metamorphosed limestones are called marbles. Fine grained deposits form chalk. Variety: onyx.

Calcium Silicate
see "Sphene"

Caliche
Caliche is naturally occurring sodium nitrate found in Chile.

Cannel Coal
Cannel Coal is a dull black coal which breaks with a conchoidal fracture and does not soil the fingers when handled. In some respects it resembles jet. It is easily cut, and will take a high polish. It contains a large proportion of volatile constituents making it suitable for gas manufacture, and it burns with a bright white flame.

Carbonaceous
Carbonaceous refers to composed chiefly of organic carbon. (i.e. carbon derived from plant and animal remains.)

Carbonates
Carbonates refers to minerals, such as calcite, where the carbonate radical (CO3) is an important constituent.

Carnallite
Carnallite is a source of potassium coumpounds and magnesium. Has a bitter salty taste and has the formulae KMgCl3ù6H2O and a relative hardness of 3.

Carnelian
Carnelian is a clear red chalcedony, a semi-precious gemstone, consisting of quartz with iron impurities which give it a translucent red colour. Carnelian is found mainly in Brazil, Japan and India.

Carnotite
Carnotite has the formulae K2(UO2)2(VO4)2ù3H2O.
It has a relative hardness of 1.
It is an ore of uranium and vandium. Found in sand, sandstones and around petrified trees. Probably formed as a deposition from meteoric waters. Strongly radioactive.

Cassiterite
Cassiterite is the principal ore of tin. It is widely distributed in small amounts but commerically available in only a few localities. It is frequently associated with wolframite. It is also found as rolled pebbles in placer deposits ('stream tin') but is usually found in veins associated with quartz, in or near granitic rocks. It has the formulae SnO2 and a relative hardness of 7.

Celestine
Celestine is a natural sulphate of strontium.

Celestite
Celestite has the formulae SrSO4.
It has a relative hardness of 4.
It often found disseminated through limestone or sandstone, or lining cavities in such rocks. Associated with calcite, dolomite, gypsum, sulphur, fluorite. Also found as a gangue mineral in lead veins. Used to prepare nitrate of strontium for fireworks and tracer bullets and in the refining of beet sugar.

Cerussite
Cerussite has the formulae PbCO3.
It has a relative hardness of 4.
It is an important and widely distributed supergene lead ore formed by the action of carbonated waters on galena in the upper zone of lead veins. Often found associated with galena and sphalerite.

Chabazite
Chabazite is a mineral of secondary origin found lining cavities in volcanic and intrusive igneous rocks. It has the formulae Ca(Al2Si4)O126H2O and a relative hardness of 5.

Chalcanthite
Chalcanthite is a minor ore of copper found only in arid regions. It occurs near the surface in copper veins and is often deposited on iron from the water in copper mines. It is used in calico printing, insecticides and for industrial purposes. It has the formulae CuSO45H2O and a relative hardness of 3.

Chalcedony
Chalcedony is a variant of quartz comprised of silica. Chalcedony was named after Chalkedon, near Istanbul. It was traditionally used for decorative objects and amultes. It has a relative hardness of 7.

Chalcocite
Chalcocite is one of the most important copper ore minerals. It occurs primarily in enriched zones of sulphide deposits. It has the formulae Cu2S and a relative hardness of 3.

Chalcopyrite
Chalcopyrite has the formulae CuFeS2.
It has a relative hardness of 4.
It is the most widely occuring copper mineral and one of the most important ore sources of that metal. Occurs as an original constituent of igneous rocks, in pegmatic dikes, and in contact with metamorphic deposits. May carry gold or silver and become an ore of those metals.

Chalcosine
Chalcosine is an important copper ore. It was discovered in the 16th century.

Chert
Chert is a mineral very similar to flint, but coarser and less uniform in colour. It is found principally in association with limestones, especially in the carboniferous limestone of Ireland where beds of it are found several hundred feet thick. It appears to have resulted from the solution and redeposition of the silica of certain kinds of fossils, particularly of sponges, with the pointed spicules of which it is often filled. Radiolarian chert is a streaky, dark-grey, brown or reddish rock which under the microscope is seen to consist of innumerable shells of Radiolaria firmly united together by a siliceous cementing material.

Chili Saltpetre
see "Cubic Nitr"

Chlorite
Chlorite has the formulae (Mg,Fe)6(AlSi3)O10(OH)8.
It has a relative hardness of 3.
It is a mineral group whose members usually exhibit a charactersitic green colour. The formula above is for 'green mica'. Distinguished from muscovite and green phlogopite by a lack of elasticity.

Chondrodite
Chondrodite has the formulae (Mg,Fe)3(SiO4)(OH,F)2.
It has a relative hardness of 7.
It is a common metamorphic mineral in dolomitic marbles.

Chromite
Chromite has the formulae FeCr2O4.
It has a relative hardness of 6.
It is slightly magnetic. The only ore of chromium. A common constituent of peridotite rocks and the serpentines derived from them. Also associated with corundum. One of the first minerals to separate from a cooling magma. Chromium is widely used in metal plating and in stainless steel.

Chrysoberyl
Chrysoberyl has the formulae BeAl2O4.
It has a relative hardness of 9.
It occurs in granite rocks, pegmatites, and in mica schists. Frequently in river sand and gravels. Serves as a gem stone: alexandrite and "cats eye" which can be of great value.

Chrysocolla
Chrysocolla has the formulae (Cu,Al)2H2Si2O5(OH)4ùnH2O.
It has a relative hardness of 4.
It is a minor ore of copper. A mineral of secondary origin, occuring in the oxidized zones of copper veins. Associated with malachite, azurite, cuprite, native copper. Named from two Greek words meaning 'gold' and 'glue'.

Chrysostom
Chrysostom is a gem stone of beryllium aluminate.

Cinnabar
Cinnabar (vermilion) is a red crystalline form of mercuric sulphide. It has the formulae HgS and a relative hardness of 3.
It often has a bright red colour. The only important source of mercury and it's found in few localities. Occurs as vein fillings near recent volcanic rocks and hot springs. Used in scientific equipment, in drugs, and with tin in silvering mirrors. Many other uses.

Clastic rock
Clastic rock refers to a sedimentary rock comprised of fragments of preexisting rocks that have been transported and deposited.

Clay
Clay refers to a soft sediment or deposit that is plastic when wet and comprised of very fine-grained materials, mainly hydrous aluminum silicates.

Cleavage
Cleavage refers to a mineral is said to possess cleavage if when it breaks it yields definite plane surfaces. Cleavage can be perfect as in micas or, in some minerals, completely lacking. Cleavage is always parallel to crystal faces.

Cleveite
Cleveite is a uranium-containing mineral, of interest owing to the fact that when heated with dilute sulphuric acid it liberates considerable quantities of occluded helium. It was this property which first led to the discovery of the element helium.

Clinozoisite
Clinozoisite has the formulae Ca2Al3Si3O12(OH).
It has a relative hardness of 7.

Cobalt glance
see "cobaltite"

Cobaltite
Cobaltite (cobalt glance) is an ore of cobalt consisting of a cobalt arsenosulphide of the formulae CoAsS and has a relative hardness of 6.
It is usually found in high temperature deposits, disseminated in metamorphic rocks, or in vein deposits with other cobalt and nickel minerals.

Coelestine
Coelestine is a mineral used in pyrotechnics and a s a source of strontium. Occassionaly it is cut and used as a gem stone. It has a relative hardness of 3.5.

Colemanite
Colemanite is a major source of borax and has a relative hardness of 5. It occurs in high temperature hydrothermal veins or disseminated in metamorphic rocks associated with other cobalt and nickel sulphides/arsenides.

Columbite
Columbite has the formulae (Fe,Mn)(Nb,Ta)2O6 and a relative hardness of 6.
It shows a bluish iridescent fracture surface. The main ore of niobium and tantalum; used in metallurgy to create heat-resistant alloys and in the rust proofing of stainless steel.

Concretion
Concretion refers to an accumulation of mineral matter when mineral particles become cemented together into an orderly, rounded form.

Contact metamorphism
Contact metamorphism refers to metamorphism resulting from the intrusion of magma which takes place at or near the contact point with the molten rock.

Copper
Copper is one of the essential metals of modern civilization. Native copper is found in copper veins but copper sulphides are the principal source ores of the metal. It has the formulae Cu and a relative hardness of 3.

Cordierite
Cordierite is found as an accessory mineral in granite, gneiss, schists, and in contact metamorphic zones. Transparent specimens of good colour have been used as a gem stone. It has the formulae Mg2Al4Si5O18 and a relative hardness of 8.

Corundum
Corundum is common as an accessory mineral in metamorphic rocks and as an original constituent of certain igneous rocks. Color differences give rise to several varieties of gem, notably, ruby and saphire. The deep red ruby is one of the most valuable gems, second only to emerald and diamond.It has the formulae Al2O3 and a relative hardness of 9.

Covellite
Covellite is an indigo-blue material found in most copper deposits, usually as a coating in the zone of sulphide enrichment. It has the formulae CuS and a relative hardness of 2.

Cristobalite
Cristobalite is present in many siliceous volcanic rocks as a lining in cavities. Upon heating to 1470 C it becomes nearly transparent. On cooling it assumes its initial white translucent appearance. It has the formulae SiO2 and a relative hardness of 7.

Crocoite
Crocoite is a rare mineral found in the oxidized zones of lead deposits where lead veins have traversed rocks containing chromite. Not abundant enough to be of commercial value although it does contain chromium. It's name is Greek and means 'saffron', an allusion to its colour. It has the formulae PbCrO4 and a relative hardness of 3.

Cross-stone
see "Harmotome"

Cryolite
Cryolite has the formulae Na3AlF.
It has a relative hardness of 3.
It often found enclosing brown siderite and grey galena. Always occurs in pegmatites where it's probably a precipitate from flouride rich solutions. Used in the manufacture of sodium salts, certain kinds of glass and porcelain, and as a flux for cleaning metal surfaces.

Crystal
Crystal refers to a solid mineral having a regular geometric shape and bounded by smooth flat surfaces(called crystal faces).

Crystal symmetry
Crystal symmetry refers to the repetitive pattern of crystal faces caused by the orderly internal arrangements of atoms within a mineral.

Cubic Nitre
Cubic Nitre (sodium nitrate, Chili Saltpetre) is a mineral found mainly in the Tarapaca district of Chile.

Cuprite
Cuprite has the formulae Cu2O.
It has a relative hardness of 4.
It is an important ore of copper. Found in the upper oxidized portions of copper veins. Commonly found in crystal forms.

Danburite
Danburite has the formulae CaB2Si2O8.
It has a relative hardness of 7.

Datolite
Datolite has the formulae CaBSiO4(OH).
It has a relative hardness of 6.
It is a mineral of secondary origin found usually in cavities in basalt lavas and similar rocks.

Delvauxite
Delvauxite is a mineral source of phosphate. It has a relative hardness of 2.5.

Destinezite
Destinezite is a mineral. It is a source of phosphate. It has a relative hardness of 3.

Detrital sediment
Detrital sediment refers to deposited rock and mineral fragments.

Diamond
Diamond has the formulae C.
It has a relative hardness of 10.
It is the hardest naturally occuring mineral and the most important of the gem stones. Occurs in pale shades of several colours, deep shades are rare. Gem quality diamond is transparent, of attractive colour, and without internal fractures or inclusions of other materials.

Diaspore
Diaspore has the formulae AlO(OH).
It has a relative hardness of 7.
It is a constituent of bauxites and a major source of aluminum.

Dike
Dike refers to a wall-like body of igneous rock that cuts across layers of surrounding rocks.

Diopside
Diopside has the formulae CaMgSi2O6 and a relative hardness of 6.
It is usually found as a contact metamorphic mineral in crystalline limestones. Transparent varieties have been cut and used as gemstones.

Dioptase
Dioptase has the formulae CuSiO2(OH)2 and a relative hardness of 5.
It is found in the oxidation zone of copper deposits, in cavities of massive copper minerals.

Diorites
Diorites are a group of rocks composed essentially of a soda-lime felspar and hornblende, embracing a wide range of types from acid to basic. Diorites were formed by cooling far below the surface and occur in the Scottish Highlands, the Channel Islands, California and other places.

Dolomite
Dolomite has the formulae CaMg(CO3)2 and a relative hardness of 4.
It's powder reacts vigorously with HCl. A potential ore of magnesium. Occurs in widely extended rock masses as dolomitic limestone. Often intimately mixed with calcite. Formed from ordinary limestone by the replacement of calcium by magnesium.

Dumortierite
Dumortierite has the formulae Al7(BO3)(SiO4)3O3 and a relative hardness of 7.
It often has a bright colour and fibrous habit. Occurs in metamorphic rocks rich in aluminum. Also in pegmatites and contact metamorphic rocks.

Dysodile
Dysodile is a yellow or green foliated mineral found in limestone.

Emerald
Emerald is a green precious stone variety of the mineral beryl.

Emery
Emery is an impure fine-grained aluminium oxide with the formulae Al2O3 employed widely as an abrasive. Typically emery consists of 60 percent corundum and 40 percent iron oxide in the form of magnetite.

Enargite
Enargite has the formulae Cu3AsS4.
It has a relative hardness of 4.
It is a relatively rare mineral found in vein and replacement deposits associated with pyrite, shpalerite, bornite, galena, chalcocite. Used as an ore of copper.

Enstatite
Enstatite has the formulae Mg2Si2O6.
It has a relative hardness of 6.
It is a common and widespread mineral. Found in mafic plutonic and vocanic rocks and often in both metallic and stony meteorites.

Epidote
Epidote has the formulae Ca2(Al,Fe)3Si3O12(OH).
It has a relative hardness of 7.
It is a common mineral. Found in regional and metamorphic rocks of mafic composition and as a product of alteration of other minerals. Sometimes used as a gemstone.

Epsomite
Epsomite has the formulae MgSO4ù7H2O.
It has a relative hardness of 3.
It has a bitter salty taste; dissolves easily in water. Occurs in delicate fibrous and capillary aggregates. Often called 'epsom salt'. Found as an efflorescent deposit on the walls of caves and sometimes in lake deposits.

Erythrite
Erythrite has the formulae Co3(AsO4)2ù8H2O.
It has a relative hardness of 3.
It often shows a vivid colour and occurs in thin crusts. An important indicator of cobalt mineral deposits.

Euclase
Euclase has the formulae BeAlSiO4(OH).
It has a relative hardness of 8.
It is a variety of berly.

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