The optical properties are the characteristic properties of minerals, thanks to which it is possible to recognize even microscopic minerals without damaging them. This is especially true for transparent stones, i.e.. transmitting light. Knowledge of the principles of crystal optics is therefore of great importance for the identification of gemstones, because it enables their colors to be identified without damaging them.

According to the electromagnetic theory of light J.. C. Maxwella (1831—1879) it consists in the propagation of electromagnetic waves with a length of 380-780 mm. The color of the light depends on the wavelength. The speed of light propagation in a vacuum is approx 300 000 km/s (close 299 790 km/s). The speed of light in the air is slightly slower; while the speed of light propagation in other transparent bodies, e.g.. in the water, glass or crystal, it is always lower than the speed of its propagation through the air. The body, in which light propagates at different speeds are defined as bodies of different optical density - optically denser and optically thinner.

The ratio of the speed of light in air vp to the speed of light in the environment under consideration v is called the refractive index n of this environment:

n = vp / v

In amorphous bodies, e.g.. in opal, and in crystals of the regular system, e.g.. in rock salt, or fluorite, light travels at the same speed in all directions. These bodies are called optically equidirectional bodies, that is, isotropic. In other crystals, however, the speed of light propagation depends on the direction - they are optically multidirectional bodies, that is, anisotropic.