Ilvaite
Ilvaite is a fairly common mineral but rarely discovered as being a gem that is faceted. Some of the mineral specimens are very collectable well-formed, shiny black, diamond-shaped crystals with sub-metallic luster. Gems are opaque black for collectors of the “black gem” category.
Ilvaite is bought at many locations worldwide including Elba, at Rio Marina and Capo Calamita; on Mt. Mulatto, near Predazzo, Trentino-Alto Adige, Italy. Very large crystals from Seriphos, Cyclades Islands, Greece; from the Kangerdluarssuk Plateau, in the Ilímaussaq intrusion, southern Greenland; fine crystals at Dal’negorsk, Primorskiy Kray, Russia; in america, very fine crystals from the Laxey mine, South Mountain, Owyhee County, Idaho; and the Prince of Wales Island, Alaska.
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | Crystals prismatic, to 25 cm; columnar, radiating, or compact massive. |
| Twinning: | None |
| Cleavage: | [001] Distinct, [010] Distinct |
| Fracture: | Irregular/Uneven |
| Tenacity: | Brittle |
| Hardness (Mohs): | 5.5 – 6.0 |
| Density: | 3.99 – 4.05 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | Iron Black, dark Grayish Black |
| Transparency: | Opaque; Translucent on thin edges |
| Luster: | Sub-Metallic |
| Refractive Index: | 1.727 – 1.883 Biaxial ( + ) |
| Birefringence: | 0.1560 |
| Dispersion: | Strong; r < v |
| Pleochroism: | Strong; X = dark green; Y = yellow-brown to dark brown; Z = dark brown |
| Anisotrophism: | Strong; color in reflected light: light gray to bluish gray, pinkish red to violet |
