Density: 3.9 to 4.7 / Hardness: 7.5
Always well crystallized in a square-based prism topped by a pyramid at each end.
Transparent to opaque, various colours, mostly dark brown. The blue colour is obtained by heating.
Under the effect of radioactive inclusions, the crystal lattice is disturbed, it becomes opaque and the density decreases to 3.3. This is the ‘metamict’ state. The lattice can be restored by heating.
Mineral accessory to eruptive rocks, pegmatites and in some metamorphic rocks and alluvial deposits.
Formerly, the colourless zircon, remarkable by its fire and its brightness, constituted a good substitute for diamond. Zircons are often heated to improve their colour.