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Physical and Chemical Properties of Stones

Agate comes in most colors. Agates range from transparent to opaque in a variety of beautiful colors. Agate presents various tints in the same specimen. The stones can be artificially stained to produce combinations of color more vivid than those found in the natural state. It is a semipellucid crystallized quartz, consisting of banded or with branching inclusions of chalcedony. Physical properties of agate are in general like those of quartz. Agate has irregular, sometimes circular bands of color and often replaces fossil wood. Many fossils are agatized material where the original organic substance has been replaced by agate while retaining the original structure. Agates are identical in chemical structure to jasper, flint, chert, bloodstone, and tiger-eye, and are often found in association with opal. The colorful, banded rocks are used as a semiprecious gemstone and for making mortars and pestles. One will often see these in beads, agate pendants and necklaces.
Chemical Formula SiO2 Hardness 7.00
Specific Gravity 2.61 Refractive Index 1.53 - 1.54

Found in abundance, in its purest form, amethyst is colorless. The finest quality amethyst is medium to medium dark in tone, vivid in intensity, and purple, reddish purple to bluish purple in hue. Heating removes the color from amethyst or changes it to the yellow of citrine. Most commercial citrine is made in this manner. Amethyst is the most valuable transparent, coarse-grained variety of the silica mineral quartz that is valued as a semiprecious gem for its violet color. It contains more iron oxide Fe2O3 than any other variety of quartz, and experts believe that its color arises from its iron content. Other theories attribute the color to contained manganese or hydrocarbons. All forms of quartz (including amethyst) are piezoelectric, making for important applications in electronics. Tourmaline is the only other gemstone that posesses this property.
Chemical Formula SiO2 Hardness 7.00
Specific Gravity 2.61 Refractive Index 1.53 - 1.54

Bloodstone is a green stone with red spots. It also occurs in shades of dark green with red, brown and multicolored spots. The iron minerals cause the deep red and brown colors. Bloodstone is dark-green variety of the silica mineral chalcedony that has nodules of bright-red jasper distributed throughout its mass. Polished sections therefore show red spots on a dark-green background, and from the resemblance of these to drops of blood it derives its name. Its physical properties are those of quartz.
Chemical Formula SiO2 Hardness 7.00
Specific Gravity 2.60 Refractive Index 1.53 - 1.54

Carnelian is a translucent form of the silica mineral chalcedony, which is a variety of quartz. Carnelian ranges in color from yellow or reddish-brown to a deep red. When it grades into brown it is known as Sard. When it contains bands of white, it is known as Sardonyx. Chemically, Carnelian is composed of silicon dioxide, SiO2, and gets its red color from hematite (iron oxide-FeO2) impurities.
Chemical Formula SiO2 Hardness 7.00
Specific Gravity 2.60 Refractive Index 1.53 - 1.54

Citrine is found pale yellow to a madeira orange in all of its glorious golden and yellow colors. The yellow color is from the presence of iron, the darker the color - the higher the grade. Citrine is transparent, coarse-grained variety of the silica mineral quartz. Citrine is a semiprecious gem that is valued for its yellow to brownish color and its resemblance to the rarer topaz. Natural citrine is rare compared to amethyst or smoky quartz, both of which are often heated to turn their natural color into that of citrine.
Chemical Formula SiO2 Hardness 7
Specific Gravity 2.6 - 2.7 Refractive Index 1.57 - 1.58

Fluorite or fluorspar, mineral composed of calcium fluoride (CaF2), the principal fluorine-bearing mineral. It occurs as cubic, isometric crystals and cleavable masses. When pure, fluorite is colorless and transparent, or translucent with a glassy luster. It often occurs with impurities which are usually hydrocarbons that make it yellow, blue, purple, green, rose, or brown. The hydrocarbons, and thus the coloring, can be removed from a specimen by heating. Several varieties exhibit fluorescence. The mineral is usually found either in pure veins or associated with lead, silver, or zinc ores. It is common in limestone and dolomites and is occasionally found as an accessory mineral in pegmatites and other igneous rocks.
Chemical Formula CaF2 Hardness 4.00
Specific Gravity 3.10 Refractive Index 1.43

The versatile garnet comes in a virtual rainbow of colors, including pink, red, purple, orange, yellow, violet, green, colorless, occasionally black, brown and many shades of red and green. The most common color of garnets is reddish brown. Bohemian garnet is deep red gemstone, Russian demantoid and African tsavorite are vibrant green. Garnets basiclly belong to the family of pyropes whose chemical composition is Mg3Al2[SiO4]3. Garnets are a group of common silicate minerals that have similar crystal structures and chemical compositions. It can be translucent to transparent, sometimes opaque with a vitreous or resinous lustre. The best known type of garnet is red semi-precious stone pyrope, one of several red gems, which the ancients used to call carbuncles. Garnets are actually one of the largest families of gemstones. Most natural garnets are mixtures of two or more of the following pure species: pyrope, almandine, spessartine, uvarovite, grossular, andradite. Garnets occur in a very wide variety of formations, colors, and clarities.
Chemical Formula X3Y2Si3O12
(X = Mg, Fe or Ca)
(Y = Al else Fe3+ or Cr)
Hardness 6.5 - 8.5
Specific Gravity 3.5 - 4.3 Refractive Index 1.78 - 1.89

Hematite is a silvery, shiny opaque stone that almost looks like metal. Hematite is heavy and relatively hard oxide mineral, that constitutes the most important iron ore because of its high iron content (70 percent) and its abundance. Hematites occur in opaque red or red-brown.
Chemical Formula Fe2O3 Hardness 6.50
Specific Gravity 5.20 Refractive Index 2.94 - 3.22

Iolite (also called dichroite or cordierite) is a transparent, violet-blue, light blue, blue, rich blue-violet stone. It is a blue silicate mineral that occurs as crystals or grains in igneous rocks, only as a result of contamination of the magma by aluminous sediment. Its chemical composition is magnesium aluminum silicate (Mg2Al3O).
Iolite has a rather extraordinary optical property. The gemstone changes colors depending upon which angle it is viewed from.
Chemical Formula Fe2O3 Hardness 7.00
Specific Gravity 2.63 Refractive Index 1.53 - 1.55

Jade is a compact, opaque gemstone ranging in color from dark green to almost white. The term is applied to specimens cut from the minerals jadeite and nephrite.
Jadeite, the less common and more highly prized of the two minerals, is a silicate of sodium and aluminum, NaAl(SiO3)2, usually containing some iron, calcium, and magnesium. It belongs to the group of minerals called pyroxenes. Jadeite crystallizes in the monoclinic system but rarely occurs in distinct crystals and is usually found in fibrous, compact, massive aggregates. The luster on fresh fracture is dull and wax-like, but polished jadeite has a vitreous luster. Nephrite, a member of the amphibole group of minerals, is a silicate of calcium and magnesium, with a small amount of iron replacing part of the magnesium. It is a tough, compact variety of the mineral tremolite. Polished nephrite has an oily luster.
Chemical Formula NaAl(SiO3)2 Hardness 6.5 - 70
Specific Gravity 3.3 - 3.5 Refractive Index 1.66 - 1.68

Jasper exhibits various colors, but chiefly brick red to brownish red. It owes its color to admixed hematite, but when it occurs with clay admixed, the color is a yellowish white or gray, or with goethite, a brown or yellow. Often jasper is found multi-colored. Chemically SiO2, jasper is opaque, fine-grained or dense variety of the silica mineral chert. Jasper, long used for jewelry and ornamentation, has a dull lustre but takes a fine polish. Its hardness and other physical properties are those of quartz.
Chemical Formula SiO2 Hardness 7.00
Specific Gravity  2.61 Refractive Index 1.53 - 1.54

Lapis lazuli (also called lazurite,) occurs in various shades of blue with some qualities being speckled with white calcite and some with yellow pyrite. The finest lapis lazuli is even blue color with little or no veining from other elements. Lapis lazuli is a semiprecious stone valued for its deep blue color. The source of the pigment ultramarine, lapis lazuli is not a mineral but a rock colored by lazurite. In addition to the sodalite minerals in lapis lazuli, small amounts of white calcite and of pyrite crystals are usually present. Because lapis is a rock of varying composition, its physical properties are variable.
Lapis Lazuli
Chemical Formula Na3CaAl3Si3O12S Hardness 5.50
Specific Gravity 2.80 Refractive Index 1.50

Malachite is an opaque, banded stone, the colors in the bands range from a very light green to almost deep green. It is a semi-precious stone and also a valuable copper ore, hydrous copper carbonate [Cu2CO3(OH)2 ] It is responsible for the green color of tarnished copper and bronze. Because of its distinctive bright green color and its presence in the weathered zone of nearly all copper deposits, malachite serves as a prospecting guide for that metal. Malachite has been used as an ornamental stone and as a gemstone.
Chemical Formula Cu2CO3(OH)2 Hardness 4.00
Specific Gravity 3.80 Refractive Index 1.85

Moonstones come in a variety of colors, ranging from colorless to white, gray, brown, yellow, orange, green, or pink. Clarity ranges from transparent to translucent.
Chemical Formula KAlSi3O8 Hardness 6.00
Specific Gravity 2.57 Refractive Index 1.52 - 1.53

Moss agate, also called mocha stone is a cryptocrystalline mineral (chalcedony) characterized by dendritic, or treelike inclusions of hornblende. The inclusions form when manganese dioxide (MnO2) separates out during the solidification of the siliceous gel, of which all agates are composed. The dendrites are usually dark green or brown, and are best seen when the grayish white translucent agate is cut cabochon and then highly polished.
Moss Agate
Chemical Formula SiO2 Hardness 6.5 - 7.0
Specific Gravity 2.6 Refractive Index 1.54 - 1.55

The color of pearls varies with the mollusk and its environment. It ranges from black to white, with the rose of Indian pearls esteemed most. Other colors are cream, gray, blue, yellow, lavender, green, and mauve. All occur in delicate shades. Cultured pearls are being produced in virtually every color of the rainbow.The chief component of the nacre that constitutes the pearl is aragonite CaCO. Pearls are formed by a mollusk consisting of the same material (called nacre, or mother-of-pearl) as the mollusk's shell. It is a highly valued gemstone. The shell-secreting cells of the mollusk are located in the mantleof its body. When a foreign particle penetrates the mantle, the cells attach to the particle and build up more or less concentric layers of pearl around it. Irregularly shaped pearls called baroque pearls are those that have grown in muscular tissue. Pearls that grow adjacent to the shell are often flat on one side and are called blister pearls. Pearls are characterized by their translucence and lustre and by a delicate play of surface color called orient. The more perfect its shape (spherical or droplike) and the deeper its lustre, the greater its value. Only those pearls produced by mollusks whose shells are lined with mother-of-pearl (e.g., certain species of both saltwater oysters and freshwater clams) are really fine pearls. Pearls from other mollusks are reddish or whitish, porcellaneous, or lacking in pearly lustre. The surface of a pearl is rough to the touch. Pearls come in a wide range of sizes. Those weighing less than 1/4 grain (1 pearl grain = 50 milligrams = 1/4 carat) are called seed pearls. The largest naturally occurring pearls are the baroque pearls; one such pearl is known to have weighed 1,860 grains.
Cultured pearl is natural but cultivated pearl produced by a mollusk after the intentional introduction of a foreign object inside the creature's shell.
Chemical Formula CaCO3 Hardness 3.00
Specific Gravity 2.70 Refractive Index 1.53 - 1.68

Peridot (also called chrysolite) is a stone with a sparkling pale green color. It occurs in lime, yellowish green, olive green or medium dark green hues. Chemically composed of magnesium iron silicate [Mg,Fe]2SiO4; Peridot (precious olivine) is a gem-quality transparent green olivine. The crystals of peridot have a vitreous lustre and conchoidal fracture. Gem-quality olivine is a mineral that composes a lot of the earth's mantel, the layer below the crust. It is also common in basalts on the moon.
Chemical Formula X2SiO4
(X = Mg
or Fe)
Hardness 6.5 - 7
Specific Gravity 3.2 - 4.2 Refractive Index 1.63 - 1.67

Quartz, second most common of all minerals, composed of silicon dioxide, or silica, SiO2. It is distributed all over the world as a constituent of rocks and in the form of pure deposits. It is an essential constituent of igneous rocks such as granite, rhyolite, and pegmatite, which contain an excess of silica. Quartz crystallizes in the rhombohedral system. The size of the crystals varies from specimens weighing a metric ton to minute particles that sparkle in rock surfaces. Quartz is also common in massive forms, which contain particles ranging in size from coarse-grained to cryptocrystalline (grains invisible to the naked eye but observable under a microscope). The luster in some specimens is vitreous; in others it is greasy or splendent (shining glossily). Some specimens are transparent; others are translucent. In the pure form, the mineral is colorless, but it is commonly colored by impurities.
Quartz crystal has applications in the electronics industry for controlling the frequency of radio waves.
Chemical Formula SiO2 Hardness 7.00
Specific Gravity 2.65 Refractive Index 1.54 - 1.55

Sodalite is a deep, rich blue stone with white inclusions typically occurring in nepheline, syenites and related rocks. It is composed of chloric sodium aluminum silicates (tectosilicates, sodalite Group � Na4Al3Si3O12Cl). Sodalite rarely forms crystals (dodecahedrons), instead it is most commonly massive. It has poor cleavage and is uneven to subconchoidal fracture, therefore making it brittle. Soluble in hydrochloric and nitric acids, it also fuses fairly easy.
Chemical Formula Na4Al3Si3O12Cl Hardness 5.50
Specific Gravity 2.27 Refractive Index 1.48

Tiger eye has a rich yellow and golden brown stripes, with a fine golden lustre when polished. It is basically a variety of crystalline quartz formed by alteration of an asbestos mineral (it being replaced by silica) known as crocidolite. When tiger eye is viewed in reflected illumination, a ripple or band of light, resembling a cat�s eye, follows the original fibrous structure of the asbestos. Tiger eye displays chatoyancy (a vertical luminescent band like that of a cat's eye � French chat � �cat�).. Tiger eye typically has lustrous alternating yellow or brown bands.
Tiger Eye
Chemical Formula SiO2
[with Na2(Mg,Fe)3Fe2Si8O22(OH)2]
Hardness 7.00
Specific Gravity 2.65 Refractive Index 1.54 - 1.55

Tourmaline is borosilicate mineral of complex and variable composition. Tourmaline is very abundant and has the best-developed crystals in pegmatites and in metamorphosed limestones in contact with granitic magmas. The colored varieties, when transparent and free from flaws, are cut as gems. Transparent crystals of tourmaline are dichroic - the depth of color varies as the crystal is turned in the light.
Another peculiarity of tourmaline is that crystal when heated acquires an electric charge and attracts small objects such as hair or small pieces of paper. Rubbing crystal imparts a similar charge.
Chemical Formula XY3Al6B3Si6(OH)4
(X = Na or Ca)
(Y = Mg, Li, Al or Fe
Hardness 7 - 7.5
Specific Gravity 3.0 - 3.3 Refractive Index 1.62 - 1.65

The color of turquoise ranges from sky blue through various shades of green to greenish and yellowish gray. The sky-blue variety of turquoise, commonly referred to as robin's egg, is the form most desired for jewelry. When excessively exposed to sunlight or heat, this variety may become dehydrated and turn green. Turquoise is opaque except in the thinnest splinters, and has a feeble, faintly waxy lustre. Turquoise is opaque and cryptocrystalline mineral composed chiefly of hydrated copper and aluminum phosphates [CuAl6(PO4)4(OH)8*4H2O]. It is a secondary mineral deposited from circulating waters, and it occurs as granular veins running through a host rock.
Chemical Formula CuAl6(PO4)4(OH)8�4(H2O) Hardness 6.00
Specific Gravity 2.80 Refractive Index 1.61 - 1.65

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