What qualifies a gem as precious or semi-precious 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.

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.

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 occur during cooling of lava where steam and other gases form bubbles. The bubbles overtaken by solidification form 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. Agate on Mohs' hardness scale is 6.5 - 7.

Amethyst is a transparent, coarse-grained variety of the silica mineral quartz that is valued as a semi-precious gem for its violet color. Amethyst ranges in color from pale lilac to deep purple. The pale varieties are sometimes called "Rose de France" and are set in Victorian jewelry. The deep colors are the most valuable, particularly a rich purple. Amethyst is the most highly valued stone in the quartz group. It is mined in Brazil, Uruguay, Bolivia and Argentina, as well as in Zambia and Namibia. Amethyst 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 between 470-750 degrees C changes the color to the yellow of citrine. Amethyst on Mohs' hardness scale is 7.

The name aquamarine is taken directly from the Latin term aqua marina, or "sea water", refers to the blue-green to deep blue beryls. The most desired color is dark blue. The coloring agent in aquamarine is iron. It is brittle and very sensitive to pressure. This gemstone is classified under the mineral group beryl. The most notable deposits are located in Brazil and spread throughout the country. Aquamarine on Mohs' hardness scale is 7.5 to 8.

Aventurine is mostly dark green with metallic iridescence caused by golden hematite leaves. Deposits are found in Brazil, India, Austria, Russia and Tanzania. Aventurine is number 7 on Mohs' hardness scale.

Blue quartz is a course-grained quartz aggregate. Deposits are found in Brazil, Austria, Scandinavia, South Africa and the United States. Blue quartz is number 7 on Mohs' hardness scale.

Calcite, which gets its name from "chalix" the Greek word for lime is a common yet amazing mineral. It comprising about 4% by weight of the Earth's crust and is formed in many different geological environments. 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. There are more than 300 crystal forms identified in calcite and these forms can combine to produce the thousand different crystal variations. Calcite is number 3 on the Mohs' hardness scale; thus, it can be scratched readily by a knife blade.

Chrysoprase is a variety of the silica mineral quartz and is the more valuable chalcedony variety. It is recognized by a lucent green color and resembles a beautiful green apple. It owes its bright apple-green color to colloidally dispersed hydrated nickel silicate. It can fade as a result of extended exposure in the light of the sun or if exposed to excessive heat. Transparency varies from translucent to opaque. Its physical properties are those of quartz. Typical occurrences are in serpentine as a secondary mineral found in cavities and veins; California and Silesia have notable deposits. Imitation chrysoprase is chalcedony dyed with nickel or chromium salts. Chrysoprase is number 6.5 to 7 on the Mohs' hardness scale.

Along with calcite and aragonite, dolomite makes up approximately 2 percent of the Earth's crust. Dolomite crystals are colorless, white, pinkish, or bluish. Granular dolomite in rocks tends to be light to dark gray, tan, or white. Dolomite crystals range from transparent to translucent, but dolomite grains in rocks are typically translucent or nearly opaque. The lustre ranges from subvitreous to dull. Dolomite, like calcite, cleaves into six-sided polyhedrons with diamond-shaped faces. However, relations between lamellar twinning and cleavage planes differ from those of calcite, and this difference may be used to distinguish the two minerals in coarse-grained rocks such as marbles. Dolomite has a Mohs' hardness of 3.5 to 4.

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.

The name hematite (Greek--blood) is derived from the fact that when cut the saw coolant becomes colored red. Optical properties are opaque with a metallic luster. The chemical composition of hematite is Iron Oxide (Fe2O3). Hematite is number 5.5 to 6.5 on the Mohs' hardness scale.

A gem-quality silicate mineral in the pyroxene family that is one of the two forms of jade. The more prized of the two types of jade, jadeite (imperial jade) is usually found as transparent-to-opaque, compact, veins, or nodules. Jadeite, sodium aluminum silicate (NaAlSi2O6), may contain a number of impurities (often calcium) that give it a variety of colours: white, emerald green, apple green, red, brown, and blue. Jadeite is a rare mineral with a number 6.5 to 7 on the Mohs' hardness scale.

An opaque variety of Chalcedony, usually red, brown or yellow and coloured by oxides of iron. It has been long used for jewelry and ornamentation. Jasper is common and widely distributed, occurring chiefly as veinlets, concretions, and replacements in sedimentary and metamorphic rocks, as in the Urals, North Africa, Sicily, Germany, and elsewhere. Jasper is number 6.5 to 7 on the Mohs' hardness scale.

a feldspar mineral in the plagioclase series that is often valued as a gemstone and as ornamental material for its red, blue, or green iridescence. The mineral is usually gray, brown or black and need not be iridescent. Labradorite is one of the more common plagioclase varieties and occurs in many dolerites, norites, and basalts.

The beautiful blues in paintings from the Renaissance are thanks to the blue of lapis lazuli, the blue rock loved by the ancients from Mesopotamia, Egypt, Persia, Greece and Rome. The name is international, from the Latin, lapis, which means stone, and from the Arabic, azul, which means blue. Lapis lazuli is 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. In addition to the lazurite minerals in lapis lazuli, small amounts of white calcite and of pyrite crystals are usually present. Lapis lazuli is still mined in Afghanistan and Chile. Because lapis is a rock of varying composition, its physical properties are variable. Lapis lazuli is somewhat porous and should be protected from chemicals and solvents. It usually occurs in crystalline limestones and is a product of contact metamorphism. Lapis on Mohs' hardness scale is 5.5.

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.

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 lustre. Sodalite is number 5.5 to 6 on the Mohs' hardness scale.

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 (less than 1% water by weight), 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 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. Different obsidians are composed of a variety of crystalline materials. Their abundant, closely spaced crystallites are so numerous that the glass is opaque except on thin edges. Obsidian in attractive and variegated colors is sometimes used as a semi-precious stone.

A striped, semi-precious variety of the chalcedony silica mineral with white, black or red alternating bands. Onyx is used in carved cameos 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. Chemically, onyx is silicon dioxide (SiO2). It differs from agate in the flatness of its layers. The chief localities of onyx are India and South America. Its properties are the same as those of quartz.

Pyrite is also known as fool's gold because it is often confused with gold. However, the actual chemical composition is iron sulphide (FeS2). It crystallizes in a cubic system, which are striated on each face and at right angles to each other. Pyrite is number 6 to 6.5 on the Mohs' hardness scale.

Quartz is the second most abundant mineral in the Earth's crust after feldspar. It occurs in nearly all acid igneous, metamorphic, and sedimentary rocks. It is an essential mineral in such silica-rich felsic rocks as granites and rhyolites. It is highly resistant to weathering and tends to concentrate in sandstones and other detrital rocks. Microcrystalline varieties of silica known as agate and jasper consist of a fine network of quartz. Metamorphism of quartz-bearing igneous and sedimentary rocks typically increases the amount of quartz and its grain size.

Quartz is piezoelectric which means it develops positive and negative charges on alternate prism edges when it is subjected to pressure or tension. The charges are proportional to the change in pressure. Because of its piezoelectric property, a quartz plate can be used as a pressure gauge, as in depth-sounding apparatus. Just as compression and tension produce opposite charges, the converse effect is that alternating opposite charges will cause alternating expansion and contraction. A section cut from a quartz crystal with definite orientation and dimensions has a natural frequency of this expansion and contraction that is very high, measured in millions of vibrations per second. Properly cut plates of quartz are used for frequency control in radios, televisions, and other electronic communications equipment and for crystal-controlled clocks and watches.

A pink mineral composed of manganese carbonate (MnCO3) that is a source of manganese for the ferromanganese alloys used in steel production. The crystal structure of many carbonate minerals reflects the trigonal symmetry of the carbonate ion, which is composed of a carbon atom centrally located in an equilateral triangle of oxygen atoms. Pure rhodochrosite is relatively rare and expensive.

The pale pink color of the silica mineral quartz found in pegmatities, which can range from transparent to translucent, is known as rose quartz. The color is a very pale and delicate powder pink, which may be due to Titanium. Transparent rose quartz is very rare and is usually so pale that it does not show very much color except in large sizes. The translucent quality of rose quartz is much more available and is used for beads, carvings, and architectural purposes. Its milky aspect is attributed to tiny needlelike inclusions of rutile, which, when oriented, give the polished stone an asterism (optical phenomenon of a star-shaped figure) like that found in sapphire, but not as sharp or intense. Rose quartz occurs in Brazil, Sweden, Namibia, California, and Maine. Its properties are those of quartz.

Smoky quartz is a brown transparent quartz that is sometimes used for unusual faceted cuts. The commercial market is limited due to the limited demand for brown gemstones. Its abundance causes it to be worth considerably less than either amethyst or citrine. Heating bleaches the stone and is often sold as citrine. Crystals of the mineral frequently contain inclusions of gas (carbon dioxide), liquid (often both water and liquid carbon dioxide), or solids (rutile). Its properties are those of quartz. This variety was sometimes known as smoky topaz in the past, which is incorrect and misleading, since the mineral variety is quartz, not topaz.

Tiger's Eye quartz contains brown iron which produces its golden-yellow color. The most important deposit is in South Africa, though Tiger's eye is also found in Western Australia, Burma, India and the U.S. Veins of parallel, blue asbestos fibres are first altered to iron oxides and then replaced by silica. The gem has a rich yellow to yellow-brown or brown color and, when polished, a fine golden lustre. Tiger's eye is number 6.5 to 7 on Mohs' hardness scale.

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. Pink tourmaline is also known as rubellite. Green tourmaline often appears black because the gem is pleochroic, which means its color is different when viewed from different angles. Tourmaline is piezoelectric. Refer to the quartz section for a full description of piezoelectricity. Tourmaline is number 7 on Mohs' hardness scale.

Encyclopædia Britannica 2003 Encyclopædia Britannica Online.

Pellant, Chris "Collecting Gems & Minerals", Sterling Publishing Co, New York.

Schumann, Walter "Gemstones of the World", Sterling Publishing Co, New York.

Holden, Martin "The Encyclopedia of Gemstones and Minerals", Michael Friedman Publishing Group Inc, New York.