Twinned crystals are not common, but the presence of polysynthetic twinning is sometimes shown by fine striations running diagonally or obliquely across the cleavage surfaces.
Crystals are usually twinned, and are often complex and difficult to decipher.
Distinct crystals are rarely met with; these are rhombohedral and isomorphous with arsenic and bismuth; they have a perfect cleavage parallel to the basal plane, c (111), and are sometimes twinned on a rhombohedral plane, e (1 ro).
Twinned crystals of quartz are extremely common, but are complex in character and can only be deciphered when the faces s and x are present, which is not often the case.
A few magnificent specimens of rock-crystal twinned according to this law have been found at La Gardette in Isere, and in Japan they are somewhat abundant.