Andradite is one of the rarest and most sought after garnets. There are no major sources of andradite and the supply is limited to small deposits.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Is a Variety of||Garnet|
|Varieties||Demantoid, Melanite, Topazolite|
|Refractive Index||1.86 - 1.95|
|Hardness||6.5 - 7|
|Specific Gravity||3.70- 4.10|
|Density||3.7-4.1; melanite about 3.9; demantoid 3.82-3.88.|
|Spectral||A strong band is visible at 4430, cutoff at the violet end of the spectrum. Sometimes (in demantoids) the Cr spectrum is visible, with a doublet at 7010, sharp line at 6930, and 2 bands in the orange at 6400 and 6220. Demantoid is red in the Chelsea filter.|
Andradite is named after the Portuguese mineralogist, d’Andrade.
Melanite has 1-5% Ti oxide; schorlomite is rich in Ti also; topazolite from Italy is yellowish-green; demantoid is rich green, colored by Cr.
This is one of the rarest and most sought after garnets. There are no major sources of andradite and the supply is limited to small deposits.
Andradite occurs in schists and serpentine rocks (demantoid and topazolite); also in alkali-rich igneous rocks (melanite and schorlomite); and in metamorphosed limestones and contact zones (brown and green colors).
San Benito County, California: topazolite (N= 1.855-1.877, S.G. = 3.77-3.81), demantoid (N= 1.882, S.G. = 3.81), and unusual catseye material. Arizona: New Jersey; Pennsylvania.
Greenland; Norway; Sweden; Uganda; Ski Lanka. Colorado: melanite.
New Mexico: in metamorphic limestones and ore deposits.
USSR: fine demantoid from the Urals. Also some (small) brown andradite.
Zaire: brown and green andradite, also some demantoid.
Ala, Piedmont, Italy: dark apple green demantoid garnet; also topazolite (yellow).
Korea: andradite, some fine green with Cr.
Monte Somma, Vesuvius, and Trentino, Italy: melanite (black).
Its dispersion is much higher than any other garnet and even much higher than diamond. The dispersion is usually masked by dark body colors but small, light colored gems are dazzling!
The variety dematoid is colored green by chromium. This gem is always in high demand.
Andradites are known for their distinctive, horsetail inclusions. (See Identifying Inclusions in our Reference Library.) They are both an aid to the gemologist and a delight to collectors.
Andradite is seldom faceted, but brownish stones up to a few carats are known. Demantoid however, is a rare but well-known gem, and is probably the most valuable of all the garnets.
Smithsonian Institution (Washington, D.C): 10.4 (USSR); also 4.1, 3.4, and 2.3.
Private Collection: 18 (sold in New York Cityl; a California collector owns a huge topazolite (green color) that would yield faceted gems over 20 carats. This crystal weighs ~1 ounce.
USSR: many fine demantoids in museum collections.
OPTICS: N= 1.88-1.94; melanite: 1.89; demantoid: 1.881-1.888; schorlomite: 1.935; topazolite (yellow): 1.887.
INCLUSIONS: Demantoid is distinguished by so-called horsetail inclusions of byssolite (fibrous amphibole); these are diagnostic for this gem. These inclusions also occasionally produce catseye gems.