Garnet comes from the Latin word, granatus which means grain. That is because many garnet deposits are small grains of red crystals in or on their host rock.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Varieties||Almandine, Almandine-Pyrope, Rhodolite, Andradite, Demantoid, Melanite, Topazolite, Andradite-Grossular, Mali Garnet, Color Change Garnet, Gadolinium Gallium Garnet, Grossular, Hessonite, Hibschite, Hydrogrossular, Transvaal Jade, Tsavorite, Malaia Garnet (Malaya Garnet), Proteus, Pyrope, Chrome Pyrope, Pyrope-Spessartine, Umbalite, Spessartite, Uvarovite, Yttrium Aluminium Garnet|
The garnets have no cleavage but display a conchoidal fracture and are somewhat brittle and tend to chip easily. The luster is vitreous, inclining to resinous in grossular, andradite, and some almandines. The hardness is 6.5-7.5 in grossular and uvarovite; 6.5-7 in andradite; and 7-7.5 in the pyralspite series.
Garnets are all isometric, and crystals show the common forms in this crystal system, such as the trapezohedron and dodecahedron. Interestingly, the most common isometric forms, the cube and octahedron, are extremely rare in garnet crystals. Garnets may also be massive, granular, and in tumbled pebbles.
Uvarovite: dark green.
Grossular: colorless, white, gray, yellow, yellowish green, green (various shades: pale apple green, medium apple green, emerald green, dark green), brown, pink, reddish, black.
Andradite: yellow-green, green, greenish brown. orangy yellow, brown, grayish black, black. The color is related to the content of Ti and Mn. If there is little of either element, the color is light and may resemble grossular.
Pyrope: purplish red, pinkish red, orangy red, crimson, dark red. Note: Pure pyrope would be colorless; the red colors are derived from Fe + Cr.
Almandine: deep red, brownish red, brownish black, violet red.
Spessartine: red, reddish orange, orange, yellow-brown, reddish brown, blackish brown. Malaya is a pyrope-spessartine from the Umba River Valley of Tanzania; the colors include various shades of orange, red-orange, peach, and pink.
A well-known commercial garnet is intermediate between pyrope and almandine. It is often said that such a garnet is a mixture of “molecules” of these garnets, whereas this really means its structure contains both Fe and Al. The intermediate garnet, known as rhodolite, usually has a distinctive purplish color.
The above variations make it easy to see why it is foolish to try to guess the identity of a garnet on the basis of color alone!
Garnet crystals are usually small, microscopic up to about 6 inches in the case of grossular. Garnets in rock, with poor external forms, may be much larger, such as the almandine from Gore Mountain, New York, which reaches a diameter of 60 cm. A few spessartines in Brazil have weighed several pounds and have retained great transparency and fine color, but these are very rare. A typical garnet crystal is about half an inch to an inch in diameter.
Garnets have several species, as well as several varieties. They are listed here, with more detailed information in the corresponding link.