AGATE 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original organic substance has been replaced by agate while retaining the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
AMETHYST 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.
BLOODSTONE 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.
CARNELIAN 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)
CITRINE 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.
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.
GARNET 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
(X = Mg, Fe or Ca)
(Y = Al else Fe3+ or Cr)
HEMATITE 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.
(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
Iolite has a rather extraordinary optical property. The gemstone changes colors
depending upon which angle it is viewed from.
JADE 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
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
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.
LAPIS LAZULI 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.
MALACHITE 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.
MOSS AGATE 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.
PEARL 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
PERIDOT 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.
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.
SODALITE 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 ï¿½
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
TIGER EYE 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 catï¿½s eye, follows the original fibrous structure of
the asbestos. Tiger eye displays chatoyancy (a vertical luminescent band like
that of a cat's eye ï¿½ French chat ï¿½ ï¿½catï¿½).. Tiger eye typically
has lustrous alternating yellow or brown bands.
TOURMALINE 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.
(X = Na or Ca)
(Y = Mg, Li, Al or Fe3+)
TURQUOISE 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.