and others show a crystalline structure comprised of several systems.
Thus calcspar is in the 2nd, or hexagonal, whilst aragonite is in the
4th, the rhombic, system, yet both are the same substance, viz.
:—carbonate of lime. Such a condition is called dimorphism ; those
minerals which crystallise in three systems are said to be trimorphous.
Those in a number of systems are polymorphous, and of these sulphur
may be taken as an example, since it possesses thirty or more
modifications of its crystalline structure, though some authorities
eliminate nearly all these, and, since it is most frequently in either
the 4th (rhombic) or the 5th (monoclinic) systems, consider it as an
example of dimorphism, rather than polymorphism.
varieties of cleavage affect the character, beauty and usefulness of
the stone to a remarkable extent, and at the same time form a means of
ready and certain identification and classification.