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Stone FAQ's
What are semi-precious stones?
What is the range of Mohs hardness scale?
What is agate?
What is amethyst?
What is calcite?
What is crisoprasio?
What is fluorite?
What is lapis lazuli?
What is leopardite?
What is sodalite?
What is obsidian?
What is onyx?
What is tourmaline?
What are semi-precious stones?
What qualifies a gem as precious and semi-precious gem is often debated, however semi-precious stones are clearly valued less than precious gems. Some examples of semi-precious gems are agate, amethyst, aquamarine, garnet, jade, jasper and lapis lazuli.
What is the range of Mohs hardness scale?
On this scale, a comparison is made of ten typical minerals, which are arranged in order from the softest to the hardest. They are: (1) talc; (2) gypsum; (3) calcite; (4) fluorite; (5) apatite; (6) orthoclase; (7) quartz; (8) topaz; (9) corundum; and (10) diamond.
What is Agate?
A common semi-precious silica mineral, that occurs in bands of varying color and transparency. Agate is essentially quartz, and its physical properties are in general those of that mineral. It is found throughout the world. Most agates occur in cavities in eruptive rocks or ancient lavas. These agates have a banded structure, successive layers being approximately parallel to the sides of the cavity. The formation of these are as follows. During cooling of the lava, steam and other gases form bubbles. The bubbles overtaken by solidification are frozen in, forming cavities. Long after the rock has solidified, alkali silicate penetrates into the bubble and coagulates to a silica gel. Soluble components of the iron rock diffuse into the silica gel and produce the regular layers of iron hydroxide. Finally the whole mass gradually hardens with loss of water and crystallization of much of the silica as quartz. During crystallization, the colored bands are not disturbed. Varieties of agate are characterized by peculiarities in the shape and color of the bands, which are seen in sections cut at right angles to the layers. Agate, with white bands alternating with bands of black, brown, or red, is called onyx. A ring or eye agate has concentric circular bands of different colors.
What is Amethyst?
Amethyst is a transparent, coarse-grained variety of the silica mineral quartz that is valued as a semi-precious gem for its violet color. It contains more iron oxide 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. Heating changes the color to the yellow of citrine.
What is Calcite?
Calcite is colorless or white when pure but may be of almost any color—reddish, pink, yellow, greenish, bluish, lavender, black, or brown to the presence of impurities. It may be transparent, translucent, or opaque. Calcite is number 3 on the Mohs hardness scale; thus, it can be scratched readily by a knife blade.
What is Crisoprasio?
Crisoprasio belongs to the family of quartz and is the more valuable chalcedony variety. It is recognized by a lucent green color. It resembles a beautiful green apple, even if this comparison is not sufficient in order to describe of the beauty. It can fade as a result of one extended exposure in the light of the sun or if subordinate to excessive heat. Transparency varies from translucent to opaque. Crisoprasio is number 7 on the Mohs hardness scale.
What is Fluorite?
Fluorite occurs most commonly as a glassy, many-hued vein mineral and is often associated with lead and silver ores; it also occurs in cavities, in sedimentary rocks, in pegmatites, and in hot-springs areas. Fluorite is used as a flux in the manufacture of open-hearth steel, of aluminum fluoride, of artificial cryolite, and of aluminum. Because of its low index of refraction and low dispersion, clear, colorless fluorite of optical quality is used for apochromatic lenses. Fluorite, or calcium fluoride (CaF2), another simple halide, is found in limestones that have been permeated by aqueous solutions containing the fluoride anion. Fluorite is number 4 on the Mohs hardness scale.
What is Lapis Lazuli?
A semi-precious stone valued for its deep blue color. The source of the pigment is not a mineral but a rock colored by lazurite (see sodalite). 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. It usually occurs in crystalline limestones and is a product of contact metamorphism.
What is Leopardite?
A variety of quartz porphyry containing quartz, orthoclase, albite, and mica. The rock has a characteristically spotted or streaked appearance due to staining by hydroxides of iron and manganese.
What is Sodalite?
Sodalite consists of a group of minerals. Different species of this group may exhibit different colors. The sodalite of most rocks occurs as irregularly shaped, translucent, bluish-colored grains with a vitreous to greasy luster. Sodalite is number 5.5 to 6 on the Mohs hardness scale.
What is Obsidian?
Natural glass of volcanic origin that is formed by the rapid cooling of viscous lava. Obsidian is extremely rich in silica (about 35 to 80 percent), is low in water, and has a chemical composition similar to rhyolite. Obsidian has a glassy lustre and is slightly harder than window glass. Though obsidian is typically jet-black in color, the presence of hematite (iron oxide) produces red and brown varieties, and the inclusion of tiny gas bubbles may create a golden sheen. Other types with dark bands in gray, green, or yellow are also known. Obsidian in attractive and variegated colors is sometimes used as a semi-precious stone.
Obsidian generally contains less than 1 percent water by weight. Under high pressure at depth, rhyolitic lavas may contain up to 10 percent water, which helps to keep them fluid even at a low temperature. Eruption to the surface, where pressure is low, permits rapid escape of this volatile water and increases the viscosity of the melt. Different obsidians are composed of a variety of crystalline materials. Their abundant, closely spaced crystallites (microscopic embryonic crystal growths) are so numerous that the glass is opaque except on thin edges.
What is Onyx?
A striped, semi-precious variety of the silica mineral agate with white and black alternating bands. Onyx is used in carved cameos and intaglios because its layers can be cut to show a color contrast between the design and the background. Other varieties include carnelian onyx, with white and red bands, and sardonyx, with white and brown bands. The chief localities of onyx are India and South America. Its properties are the same as those of quartz.
A form of quartz, onyx is a chalcedony silica mineral in which layers of white alternate with layers of black or red. Chemically, onyx is silicon dioxide (SiO2). It differs from agate in the flatness of its layers. There are many varieties of onyx. Some onyx classed as semi-precious gemstone is used in cameos and other jewelry.
What is Tourmaline?
A complex crystalline silicate containing aluminum, boron, and other elements, used in electronic instrumentation and, especially in its green, clear, and blue varieties, as a gemstone.
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