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. Biaxial stones are trichroic and three colours may be seen.
Heat is conducted differently in various minerals according to their crystal system.
This is used in Thermal Conductivity instruments to differentiate diamond which conducts heat very well from its simulants and imitations. Some instruments use it to identify other gemstones but they are expensive and of value only when used with care and some gemmological knowledge. The use of standard stones is suggested and drafts to be avoided as they can change the readings. At its simplest this is the temperature test using tongue or lips for glass and plastic. Atomic structure and the related crystal structure influence electrical properties.