|Gem Stone||Mohs (Hardness)||About & Occurrence|
Crystalline quartz in shades of purple, lilac, or mauve is called amethyst, a stone traditionally worn to guard against drunkenness and to instill a sober and serious mind. Amethyst is dichroic, showing a bluish or reddish purple tinge when viewed from different angles. Usually faceted as a mixed step cut, amethyst has distinctive inclusions that look like tiger stripes, thumbprints or feathers. Some amethyst is heat treated to change the color to yellow, producing citrine, Crystals that are part citrine and part amethyst are called amertrine.
Amethyst is found in alluvial deposits or in geodes. Some of the largest geodes containing amethyst are in Brazil. Amethyst from the Urals (russia) has a reddish tinge; Canadian amethyst is violet. Other localities include Sri Lanka, India, Uruguay, Madagascar, the United States, Germany Australia, Namibia, and Zambia.
With a value of only 5 on the Mohs scale of hardness, apatite is seldom faceted as a gemstone, except for collectors. However, when cut correctly, stones are bright with strong colors. Transparent to opaque, apatite occurs as colorless, yellow, blue, violet or green hexagonal prisms or tabular crystals.
Apatite is an abundant mineral found in many types of rock, but most gem-quality material is associated with pegmatites. Blue apatite from Myanmar is strongly dichroic, showing colorless or blue when viewed from different directions. Fibrous blue apatite from Myanmar and Sri Lanka may be cut en cabochon to show a cat's-eye. Caroyant stones are also found in Brazil, along with yellow, blue and green varieties. Other localities include the Kola Peninsula (Russia), Canada, East Africa, Sweden, Spain and Mexico.
Also called cornelian, this translucent, reddish orange variety of chalcedony was once thought to still the blood and calm the temper. Its various shades of red are due to the presence of iron oxide. Stones may be uniformly colored or faintly banded.
The best carnelian is from India, where it is placed in the sun to change brown tints to red.
Citrine is the yellow or golden yellow variety of quartz. The yellow coloration, due to the presence of iron, is responsible for the name, derived from the word citrus. Natural citrine is usually a pale yellow, but rare; most citrine on the market is heat-treated amethyst.
Gem-quality citrine is extremely rare. The best material is found in Brazil, Spain, Madagascar and the former USSR. Citrine has been used to imitate topaz and was once called Brazilian topaz.
Pryope comes from the Greek word pryopos, meaning fiery and was the fashion stone of the 18th and 19th centuries, especially the "Bohemian Garnet.". Pryope is found in volcanic rock and alluvial deposits and may, along with certain other minerals, indicate the presence of diamond-bearing rocks. Localities include the USA (Arizona), South Africa, Argentina, Australia, Brazil, Myanmar, Scotland, Switzerland and Tanzania.
Almandite is generally darker red than pryope. Its name is derived from the town in Asia Minor. Deposits are found in Brazil, India, Madagascar, Sri Lanka, Czech Republic, Austria and the United States.
Rhodolite in composition is between Almandite and pryope and is a purplish red or rose color.
Spessartite is bright orange when pure, but an increase in iron content makes the stone darker orange to red. Deposits are found in Burma (Myanmar), Brazil, China, Kenya, Madagascar, Sri Lanka, Tanzania and the United States.
Hessonite is orange-brown in color due to manganese and iron inclusions. Occurrences of Hessonite are in Sri Lanka, Brazil, India, Canada, Madagascar, Tanzania and the United States.
Jasper is a massive, fine-grained, opaque variety of chalcedony, believed to protect against sight defects and drought. It occurs in shades of brown, grayish blue, red, yellow and green, and mixtures of these. "Orbicular" jasper has white or gray, eye-shaped patterns surrounded by red jasper. "Ribbon" jasper is striped and used in carvings, cameos, and intaglios, which show off its layered structure. Hornstone is a gray variety.
Red jasper occurs in India and Venezuela; various colors occur in the United States, especially orbicular jasper in California; red and green ribbon jasper occurs in Russia. It also occurs in France and Germany.
Labradorite is the plagioclase feldspar that is most commonly faceted as a gemstone. It may be orange, yellow, colorless or red, but the material that shows a play of color, or "schiller," is the most popular for use in jewelry.
Labradorite occurs in metamorphic and igneous rocks in Labrador (Canada), Finland, Norway and the former USSR.
Moonstone is the opalescent variety of orthoclase, with a blue or white sheen (or "schiller"), rather like the shine of the moon. This is caused by the reflection of light from the internal structure, made up of alternating layers of albite and orthoclase feldspar. Thin albite layers give an attractive blue; while thicker layers produce a white "schiller." Stones of large size and fine quality are rare.
The best material is from Myanmar and Sri Lanka. Other localities include India, Madagascar, Brazil, the United States, Mexico, Tanzania and the European Alps.
Gem-quality specimens of the mineral olivine are called peridot by gemologists. Peridot has an olive- or bottle green color that is due to the presence of iron and a distinctive oily or greasy luster. It has a high bi refraction, so doubling of the back facets can easily be seen in larger specimens when viewed from the front. Good-quality crystals are very rare.
Sources of peridot include St. John's Island (Egypt), China, Myanmar, brazil, Norway, the United States (Arizona and Hawaii), Australia and South Africa.
Colorless and transparent, rock crystal is the most widely distributed variety of quartz, one of the most common minerals of the Earth's crust. The crystals are usually found as colorless hexagonal prisms, with pyramidal ends and striations perpendicular to their length; they are often twinned. Cleavage is poor and fracture conchoidal.
Although found worldwide, the most important sources of rock crystal are in Brazil. Other localities include the Swiss and French Alps, where fine crystals occur and Madagascar, the former USSR and the United States.
Pink or peach-colored quartz is called rose quartz and is mainly used in decorative carvings. Its color is thought to be due to the presence of small amounts of titanium. Crystals of rose quartz are very rare; more usually massive lumps are found, which can be carved or cut en cabochon or as beads. Transparent material is uncommon; it is usually cloudy or cracked, partly because it is so brittle. Rutile inclusions in rose quartz may produce a star effect when the stone is cut en cabochon.
Rose Quartz is found in pegmatites The best material is from Madagascar, but Brazil produces a greater quantity. Other sites are the United States (Colorado), the former USSR, Scotland and Spain.
Ruby - the name given to red, gem-quality corundum - is one of the best gemstones for jewelry settings. Rubies may be any shade of red, from pinkish to purplish or brownish red, depending on the chromium and iron content of the stone. Frequent twinning of the crystals makes the material liable to fracture, yet ruby is a tough mineral, second only to diamond in hardness. Crystal prisms are hexagonal with tapering or flat ends. As the crystals grow, they form new layers, and, depending on the geological conditions and minerals present, color variations called zoning occur.
Rubies occur worldwide in igneous and metamorphic rocks, or as water worn pebbles in alluvial deposits. The finest stones come from Myanmar; those form Thailand, the primary source, are brownish red; Afghanistan, Pakistan and Vietnam yield bright red stones; those from India, the United States (North Carolina), Russia, Australia and Norway are dark to opaque.
Brown quartz includes crystalline quartz of a light brown or dark brown color, grayish brown "smoky" quartz and the back variety called morion. Brown or smoky quartz from the Cairngorm Mountains of Scotland is called cairngorm. When irradiated, colorless quartz may change color to grayish brown, suggesting that brown quartz may have been formed by natural radiation within the ground. Brown quartz crystals are hexagonal prisms with pyramidal ends, in which inclusions of the mineral rutile may be present.
Crystals weighing as much as 650 lb (300 kg) have been found in Brazil. Other localities include Madagascar, the Swiss Alps, the United States (Colorado), Australia and Spain. Much of the smoky quartz on the market is in fact irradiated rock crystal. Brown quartz has been confused with andalusite, axinite idocrase and brown tourmaline.
Tourmaline Group - Even though tourmaline has been known since antiquity in the Mediterranean,region, the Dutch imported it only in 1703 from Sri Lanka to Western and Central Europe. They gave the new gems a Singhalese name, Turamali, which is thought to mean "stone with mixed colors."
Popular varieties of tourmaline are: Dravite, Achroite, Watermelon, Schorl and Green and Yellow Tourmaline. Tourmaline can be found in a rainbow of colors from blues, reds, yellows, greens, pinks and everything in between.
One of the first gemstones to be mined, turquoise has long been prized for its intense color, which varies from sky blue to green depending on the quantities of iron and copper within it. Turquoise is commonly found in microcrystalline, massive form, usually as encrustations, in veins or as nodules. It is opaque to semi translucent, light and very fragile, with conchoidal fracture. Some material is very porous, leading to fading and cracking, so it may be impregnated with wax or resin to maintain its appearance.
Sky blue turquoise from Iran is generally regarded as the most desirable, but in Tibet a greener variety is preferred. Localities in Mexico and the United States produce a greener, more porous material that tends to fade more quickly. Other localities include the former USSR, Chile, Australia, Turkestan and Cornwall (England).
Turquoise has been thought to warn the wearer of danger or illness by changing color. It has been imitated by stained howlite, fossil bone or tooth, limestone, chalcedony, glass and enamel. In 1972, an imitation turquoise was produced in France by Gilson.