Most collectors say that their interest in minerals is largely due to their beautiful and striking colours. But pick up a specimen, look at it closely, and you are very likely to see several other optical effects that give it even more beauty and appeal.

Transluscency

This is the property of some minerals to let through light. Only the Sulfides and Oxides are truly opaque. Most other minerals show some degree of transluscency, depending partly on thickness, but mostly on inclusions or flaws that scatter the light and block its passage. Thus almost any mineral can under ideal circumstances be almost or entirely transparent.

Fluorescence

This is the ability of some minerals to change invisible ultraviolet light to visible light. It happens because electrons in the mineral are given energy by the incoming light which gives them a kick into a higher energy state. Here, the electrons are unstable and sooner or later will lose their energy and fall back into the original state. Becuse some of this released energy goes into producing vibration within the structure, the light which emerges is less energetic and hence shines as visible light. Interestingly the fluorescence of many substances is due not to the substance itself but to some chemical impurity or a defect in its crystal structure. For example, pure zinc silicate does not fluoresce at all, but if a bit of manganese is added the substance will glow yellow-green. These materials are called activators. Significantly many impurities destroy fluorescence by promoting loss of energy that do not involve the emission of light. Typically the energy is wholly dissipated as vibrations. Among the most notorious poisoners of fluorescence are iron, and copper.

Phosphorescence

This is almost identical to fluorescence but is much rarer. In this instance the light continues to shine for some time after the stimulating source is turned off. It happens because the excited electrons are more stable in their higher energy state, and only slowly decay into their original state.

Thermoluminescence

This is a light effect whereby a mineral will glow when gently heated.

Triboluminescence

A similar effect, happens when some minerals give off flashes of light when stroked with a metal point. Percussion Figures are six-pointed stars shown by micas when struck by a light blow from a needle. The rays naturally follow crystal directions in the mica.

Asterism

The effect is characterised by a six-rayed star shown by some minerals when polished and then a point light source is shone through them.

Labradorescence

The effect is seen as a multicoloured sheen seen in feldspars when light is reflected at a particular angle. The effect is caused by refractions and reflections of light between closely lying sheets within the crystal structure.

Pleochroism

The effect is shown by some minerals where light travelling in one direction through a crystal is absorbed differently from light travelling in another. The effect is most commonly seen as a difference in colour between the two crystal axes. Tourmalines are good examples of this effect. When the effect takes place on two crystal axes it is referred to a Dichroism and the mineral is Dichroic. When it happens in all three crystal directions it is known as Pleochroism and the mineral is Pleochroic.