Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

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MALI GARNET

Mali Garnet, one of the rarer varieties within the garnet group, is a mixture of the species grossular and andradite (sometimes, therefore, called "grandite"). The entire garnet group is a solid solution series of silicates, which means the crystal structure is basically the same throughout the group, but some of the chemical element substitutions in the crystal "unit formulas" differ by species and variety. At certain key points throughout the series, designated "species" names are given, such as pyrope, almandite, Spessartite, grossular, etc. These species exhibit different optical and physical properties such as refractive indices, colors and densities, due to their differing chemistries. Each is a silicate of aluminum, chromium or iron combined with magnesium, manganese, iron or calcium.

In truth, then, a nearly infinite array of named, and as yet unnamed, garnet varieties exist in Nature as mixtures between various garnet species. Two of such mixed parentage varieties that have come to recent popularity are rhodolite and malaya.

[Also hybrids: rhodolite garnet (almandine and pyrope), "malaya" garnet (pyrope, almandite, Spessartite and grossular)]

In terms of Mali's own relatives: Tsavorite and hessonite are two well known color varieties of grossulars, and demantoid is the star andradite.

[Representatives of the parent species of Mali garnet: hessonite garnet (grossular), demantoid garnet (andradite)]

The name, Mali, was conferred on this previously unknown, brilliant, dispersive, yellow, yellow-green or brown garnet which was discovered in 1994, based on the country where the specimens were being mined in West Africa. Mali is still the only known source. Gemological study has proven that the new gem's composition is intermediate between that of grossular (calcium aluminum silicate) and andradite (calcium iron silicate ). Although Mali garnets are predominantly grossularite, the smaller, variable admixture of andradite changes its characteristics in at least one important way. The hybrid, Mali, seems to favor its grossular parent in terms of refractive index (1.77 in most specimens) hardness (7), and clarity, but derives its superb dispersion and luster from its andradite heritage.

Most rough is mined from eluvial deposits, and shows the rounded waterworn shape typical of that type of source. A great deal of material was produced the year following its discovery -- the although the popularity of this pretty stone soared, prices which were initially very high began to drop off. In recent years much smaller amounts have been produced, which has resulted in with a consequent increase in price. As large size rough is rare in this variety, price per carat increases dramatically with size.

[Typical shapes and colors of Mali rough: Image courtesy of www.EGEM.com]

Colors range from yellow to greenish yellow to yellowish brown to brown. Cut stones are remarkable for their brilliance, luster and most notably their dispersion when body color is light enough to let it show. As with all garnets, no special care is necessary in setting or wearing and they are suitable for all jewelry uses. No enhancements or synthetics are known in the market.

Value:

No doubt the high end of this variety's value range is reserved for the extremely rare chrome green color. In general yellow to yellow green to green yellow stones go for higher prices than brownish yellow and brown stones. As with any gem, size, clarity and cut affect the value dramatically, with dispersion in this case being a special value factor.

 

Gemological Data: (varies with composition)

Makeup: Calcium, iron/aluminum silicate

Crystal Structure: Cubic

Hardness: approximately 7

Luster: vitreous to subadamantine

Density: above 3.49, below 3.85

RI: about 1.77

DR: None

Dispersion: above .028 below .057

Cleavage: none
Stones Currently Available:

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Go to: Homepage -- what's new at ACS -- monthly specials and discounts -- "buried treasures" -- BWS/FS jewelry designs -- gem topic of the month -- gem topic archive -- birthstones -- ask Barbara -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- setting these gems -- free gemology course