THE COLOUR OF A CRYSTAL can be its most striking feature. The causes of colour are varied and many minerals occur in a range of colours. Something looks a particular colour largely due to your eye and brain reacting to different wavelengths of light. When white light (daylight) falls on a crystal some of the wavelengths may be reflected, and some absorbed. If some are absorbed, those remaining will make up a colour other than white because some of the wavelengths that make up white light are missing. Sometimes light is absorbed and re-emitted without changing and the mineral will appear colourless.
Some minerals are nearly always the same colour because certain light-absorbing atoms are an essential part of their crystal structure. These minerals are described as Idiochromatic. For example, copper minerals are nearly always red, green, or blue according to the nature of the copper present.
A large number of minerals occur in a wide range of colours caused by impurities or light absorbing defects in the atomic structure. For example, quartz, diamond, beryl, and corundum can be red, green, yellow, and blue. These minerals are described as Allochromatic.
The colour in some minerals is really a play of colours like that seen in an oil film or soap bubble. This may be produced when the light is affected by the physical structure of the crystals, such as twinning or cleavage planes or by the development during growth of thin films. Microscopic intergrowths of plate-like inclusions also cause similar light interference.
WHAT IS IT?" This is the first question to ask about a mineral, crystal, or gemstone. In order to identify a crystal it is necessary to test its properties. Most minerals have fixed. Or well defined chemical compositions and a clearly identifiable crystal structure. These give the mineral a characteristic set of physical properties. Colour, habit, cleavage, and surface features can be studied using a hand lens, but in most cases this is not enough for positive identification. Other properties such as hardness and specific gravity (sG) can be studied using basic equipment, but sophisticated instruments are needed to investigate optical properties, atomic structure, and chemical composition fully.