Demantoid

Demantoid is the fabled emerald-green gem variety of andradite, the calcium-iron member of the garnet group, with the chemical formula Ca₃Fe₂(SiO₄)₃. Trace amounts of chromium tint the crystal its coveted grass-to-forest green, while the mineral's cubic structure and lack of cleavage give it a Mohs hardness of 6.5–7 and an exceptionally high refractive index and dispersion — greater than diamond itself, which is exactly why 19th-century mineralogists coined the name from the Greek word for "diamond-like." Well-formed dodecahedra and trapezohedra of demantoid rank among the most desirable crystal habits in the entire garnet family, prized as much by systematic mineral collectors as by gem cutters.

The type locality and still the benchmark source is Russia's Ural Mountains, especially the historic deposits around Nizhny Tagil, the Sysert district, and the Bobrovka River, first worked in the mid-19th century and famously used by Carl Fabergé and Tiffany & Co. Classic Uralian specimens are prized for their radiating byssolite (fibrous amphibole) inclusions sprouting from a black chromite core, the so-called "horsetail" pattern that instantly authenticates Russian material. Since the 1990s, Namibia's Green Dragon Mine and other deposits in the Erongo region near Karibib have become the world's leading commercial source, yielding cleaner, often larger crystals with a brighter, more purely green tone, typically formed in skarn contact zones rather than the ultramafic serpentinite settings of the Urals. Other notable sources include Antetezambato and various localities in Madagascar, producing yellow-green to deep-green stones sometimes showing Russian-style inclusion patterns; Iran; Val Malenco in Lombardy, Italy, source of fibrous-matrix specimens on chrysotile asbestos; and smaller occurrences in Pakistan, Afghanistan, Eritrea, Sri Lanka, and California.

Fine demantoid crystals over two carats are genuinely rare, and museum-grade Uralian pieces with sharp horsetail inclusions can command prices rivaling top emeralds. Collectors distinguish demantoid from its sister andradite varieties — the yellow topazolite and opaque black melanite — and from other green garnets like uvarovite and tsavorite, making a well-documented, locality-labeled demantoid specimen, whether a lustrous Uralian crystal on matrix or a gemmy Namibian trapezohedron, a centerpiece for any serious garnet or Ural mineral collection.