Gemstones A through G

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Amazonite: An opaque light blue-green stone (a feldspar mineral).
Russian Amazonite is a deeper teal with gray and white striations.
Hardness: 6-6.5
Care: Clean with warm soapy water. Do not use a steamer or ultrasonic cleaner. May be damaged by scratches, sharp blows, abrasives, acids, and other chemicals.

Amethyst: The most prized variety of macro crystalline quartz. No standard grading system - value varies depending on clarity of the stone and desired intensity of the color, which ranges from pale mauve to almost black.
Hardness: 7
Care: May be cleaned with an ultrasonic cleaner, or warm soapy water and a soft brush. May fade with prolonged exposure to sunlight.

Apatite: A calcium phosphate - typically green, but may be blue, yellow, reddish-brown, violet, yellow-green, colorless, or multicolored. Sometimes chatoyant (cat's eye effect).
Hardness: 5
Care: Should be treated gently due to its relative softness. Very heat sensitive (similar to opals). Clean with mild soap, lukewarm water and a soft cloth. Unsuitable for steam or ultrasonic methods.

Aquamarine: A variety of beryl (other beryls include emerald and morganite). The blue or blue-green color derives from traces of iron and ranges from very pale to moderately dark. Sometimes heat treated to remove the green tint.
Hardness: 7.5-8
Care: Generally trouble free, may fade in prolonged sunlight. Normal gemstone care.

Aragonite: A form of calcium carbonate. May be white, red, brown, yellow, green, or blue.
Hardness: 3.5-4
Care: Avoid ultrasonic or steam cleaning. Avoid even weak acids.

Aventurine: A macro crystalline quartz with inclusions of mica or iron oxide which generate a sparkling effect. It occurs naturally in shades of red, orange, brown, blue and green, but is also commonly dye-enhanced.
Hardness: 7
Care: Normal gemstone care. May fade in prolonged strong sunlight.

Calcite: A form of calcium carbonate - occurs in a huge array of shapes and colors.
Hardness: 3
Care: Cool soapy water (non-acidic pH). Avoid ultrasonic or steam cleaning. Readily dissolves in acids.
Carnelian: A cryptocrystalline quartz. Heat treated (in this case, a tradition thousands of years old) to achieve the fiery orange red color. Untreated forms are cloudier and range in hue from almost white to dark red.
Hardness: 7
Care: Normal gemstone cleaning methods. Possible color change with heat.

Chalcedony: A broad term for the cryptocrystalline quartzes, which include agate, jasper, carnelian, chrysoprase, onyx, and bloodstone. Although there may be overlap at times, as a gemstone descriptor, the term generally refers to a translucent chalcedony with a solid color and nearly wax-like luster. The most popular naturally occurring colors are blue (in a variety of hues) and purple. (Pink, yellow, orange and red also occur.) Although the natural color spectrum can be quite brilliant, it is sometimes enhanced.
Hardness: 7
Care: Generally trouble free and safe for normal gemstone cleaning methods. Some sources suggest possible color change with prolonged exposure to strong sunlight.

Chrysoprase: One of the most valuable of the quartz gemstones, a rich apple green form of cryptocrystalline quartz. Highest quality forms include little or no brown matrix and are more translucent.
Hardness: 7
Care: Can fade with sunlight and heat. Can dehydrate, losing translucency and luster - this may be corrected by wrapping in wet cotton.

Citrine: A form of macro crystalline quartz. Much of the citrine on the market is actually amethyst or smoky quartz, which turn to citrine with the application of specific amounts of heat. Heat treated forms, particularly those made from amethyst, tend to have a slightly reddish tint (sometimes called "Madeira citrine"), whereas naturally occurring citrine, which is uncommon, tends to be a much paler lemon yellow.
Hardness: 7
Care: Can change color with prolonged exposure to strong light or heat. Generally safe for normal gemstone cleaning methods.

Emerald: A variety of beryl (other beryls include aquamarine, heliodor and morganite). The prized green color is caused by small amounts of chromium and iron. Emeralds are routinely treated with oil to fill the fissures which are common to the stone - any emerald which is not specifically designated "un-enhanced" may be assumed to be so treated.
Hardness: 7.5-8.
Care: Clean with a soft dry cloth. Soapy water or other forms of cleaning may damage or remove the oil.

Garnet: Refers to a group of silicate minerals with similar crystal structures.
Types:
Almandine and pyrope types blend to form the common dark red garnet. May be dye enhanced.
Spessartite - somewhat rare - the most valuable types are a bright orange red.
Grossular - light to medium tone, found in every color except blue
Hessonite - an orange to pinkish orange variety of grossular
Tsavorite - deep chromium green variety of grossular
Hydrogrossular - translucent to opaque, commonly blue-green, used as a jade substitute.
Uvarovite - rarest of the garnet family - a dark rich green.
Andradite - one of the rarest garnets - dematoid is the highly sought after green variety
Hardness: 6.5-7.5
Care: Generally sturdy and trouble free - normal gemstone care.

Gaspeite: A rare, apple-green form of calcite. (See entry for calcite above). Some vendors appear to be passing off a dyed white stone similar to chalk "turquoise" as gaspeite.

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