Natural crystal andradite was named to honor the Portuguese mineralogist, Jose Bonifacio de Andrada e Silva 91763-1838). This is one of the most sought-after garnets. It is composed from calcium iron silicate. The crystal system is cubic and the class is silicates with isolated silica units. Supply is restricted to fairly small deposits. Natural crystal andradite garnet contains calcium and iron. Yellow natural crystal andradite is called topazolite, and vibrant green natural crystal andradite is known in the trade as demantoid. Melanite is a black (occasionally dark red), titanium-rich variety of andredite garnet. Brown natural crystal andradite, the commonest color, is rarely faceted or used in jewelry. A few natural crystal andradites are found with an iridescent sheen on some faces, these stones come from Mexico and Japan. This stone has none both birefringence and cleavage and the hardness stopped at 6.5.
Some transparent green natural crystal andradite is described as demantoid, or diamond-like, because of its high dispersion (0.057). The habit is dodecahedral or trapezohedral crystals, often showing growth zoning. Faceted demantoid gems over two carats are extremely rare. The natural gemstone qualities of topazolite garnets are a good match for demantoids, but because they are very small, they are not practical for jewelry use. Natural crystal andradite is rare to find faceted gems larger than two or three carats. The color is available in bright green, black, yellowish brown, red, greenish yellow, gray. Natural crystal andradite has luster that vitreous to adamantine with conchoidal fracture. In green natural crystal andradite the refractive index is between 1.888-1.889 and the specific gravity is about 3.82-3.86.
Natural crystal andradite’s dispersion is much higher than that of any other garnet and significantly higher than that of a diamond. Dark body colors normally mask the dispersion, but small, pale, and well-cut gems are impressively fiery. Demantoid is the most valuable of the natural crystal andradite garnets. The finest material is found in alluvial goldbearing deposits in the Ural Mountains in Russia, and also in metamorphic formations, but other good sources are located in Namibia, and Iran. Other notable sources of natural crystals andradite are Italy, Mexico, Japan, and USA (California and Arizona). Demantoid’s color can vary from a very pale green to a rich, medium green colored by chromium. Materials from Russia (and other sources) have characteristics “horsetail” inclusions.
