Sphalerite usually appears associated to calcite or dolomite, although it can also be found associated to pyrite, fluorite and quartz. The sphalerite associated to calcite and dolomite gives rise to beautiful large samples. Sphalerite crystals range in sizes from tiny specks to crystals more than 6-8 cm across. The largest crystals can reach up to 20 cm (Gómez-Fernández et al., 2000).
Large sphalerite crystals aggregate from Aliva mine. 26 x 15 x 9 cm. 5.7 kg (28,500 ct)
(1€ coin diameter = 2.3 cm)
Same sample with rear illumination showing transparency and color of one of the crystals.
Raúl Sanabria Orellana and José Ramón García Álvarez described in detail sphalerite crystals in the Spanish mineralogical magazine: Revista de Minerales (nº 2, 2005). According to the authors, sphalerite crystals have a complex development, generally very twinned and deformed, which makes very difficult their accurate description. Sphalerite belongs to the cubic crystal system, class 4-3m, space group F43m (in Hermann-Mauguin notation), or "216" in the International Tables for Crystallography. Typical crystal forms for Aliva sphalerites are complex combinations of rhomododecahedron, tetrahedron, trigonal tristetrahedron and cube faces.
Combination of different forms in sphalerite crystals (drawing by www.smorf.nl)
Moreover, the simple crystals a very rare at Aliva mine, almost all of sphalerite crystals are twinned on (111) planes, usually forming polysynthetic twins of multiple individual crystals. The twinning can be easily observed in cleavage planes of almost all sphalerite samples; the surfaces of exfoliation appear striated due to polysynthetic twinning.
Examples of twinned sphalerite crystals (drawing by www.smorf.nl). Twinning planes are marked in yellow.
Polysynthetic twinning observed on cleavage plan of sphalerite crystal.
During the faceting process, the twinning can be easily observed. The difference of the crystallographic orientation of twinned crystals gives them varied appearance on grinded surfaces. These differences are usually erased during the polishing process, but sometimes it can be difficult to get good polish on such facets.
Crystal twinning observed on grinded surface during faceting process (left). Same stone after polishing first facets (right).The twins became invisible on polished facets, while in the unpolished parts they can be easily seen.