A crystal may be cleaved in directions related to the external form or to a possible crystal form for the mineral. Sometimes two distinct minerals can have the same chemical composition with their differing properties being due to their different crystal structure.
Crystal structure affects mineral properties more than their chemical nature.
In uniaxial crystals the isotropic direction is that of the main crystal axis. Thus depending upon the direction one looks at the stone relative to the crystal and optical axes a different colour is seen. Both colours are often present at the same time however and it requires a dichroscope to separate the colours to see them. The dichroscope allows each ray s colour to be viewed separately and at the same time to compare them. Uniaxial gemstones are dichroic and two colours may be observed.