Knowledge

COLOUR-CHANGING STONES

Alongside truly multicoloured variations, some gemstones possess definite effects that suggest a colour change or at least a change in nuances of colour.

Thus some gemstones can appear to be vivid in colour when viewed from some angles, while appearing pale from others. Some stones appear to change their colour depending on the light source under which they are viewed.

Various optical effects are responsible for such behaviour.

PLEOCHROISM

Pleochroism is an optical phenomenon in which a gemstone has different colours when viewed from different angles.

The reason for this behaviour lies in the crystalline structure of the stone, which causes the light passing through it to be broken into the colours of the visible light spectrum. This refracted light is then reflected and refracted again, with some colours being absorbed by the stone.
Crystals with an optical axis cause the gemstone to show two main colours. This is called dichroism.
Crystals with two different axes can appear to have three different colours – trichroism.

Strong colour changes can often be seen in tanzanite (crimson, dark blue, yellow-brown) as well as spodumene (pink variation: kunzite), but other gemstones, such as rubies, emeralds and sapphires can also show some colour-change properties.
Depending on the strength of the effect, one differentiates between strong, definite and weak pleochroism.

If a gemstone has some pleochroism, it is important that this is taken into account during cutting.

Farbwechsel Saphir

Alexandrite-Effect

Stones that show no or only weak colour change are not considered to be alexandrite and are thus difficult to sell.

The alexandrite effect refers to the apparent change of colour of a mineral when viewed under different light sources and is caused by the crystal lattice of the gemstone. Depending on the light source, different frequencies of the light spectrum are absorbed, making the stone appear, for example, more red or more green. This phenomenon is thus caused by the absorption of light.

This kind of colour change can be seen most strongly in alexandrite, the gemstone after which the effect is named. When viewed in daylight the gemstone appears green-blue, while under incandescent light or natural candlelight it is red to violet.

If you wish to buy an alexandrite as an investment, it is important to pay attention to its colour change properties. Large stones with high clarity and strong colour change are sought-after. Such stones are extremely rare and thus command very high prices.

The amount of colour change from green to red is measured in percent, with the best value being 100%. Do not buy a stone with a colour change of under 50%; from 80-90% is excellent.

This effect is only found in a few gemstones: alongside chrysoberyl (the generic mineral group for alexandrite), it can also be observed in garnet, sapphire, tourmaline and spinel.

FLUORESCENCE

Fluorescence is an optical phenomenon that can be observed when an object is lit by an ultraviolet light source. It is the emission of visible light from the gemstone.

Fluorescence in diamonds is absolutely undesirable, as a diamond must always have the same colour, no matter what the lighting conditions. Only buy diamonds that have a certificate that shows they have no fluorescence.

In contrast to diamonds, fluorescence in rubies is highly regarded; especially rubies from Burma show a high level of fluorescence, due to their lack of iron and minimal chromium content. This can be seen by candlelight, when such a ruby develops a deep, glittering red colour.

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Dr. Thomas Schröck
The Author:

Dr. Thomas Schröck

The founder and managing partner of THE NATURAL GEM has been active in international gemstone trading for 30 years. As a doctor of economics and a certified gemmologist in Switzerland, Germany and the USA, among other countries, he is one of Europe’s leading experts on naturally-coloured, untreated gemstones and investments in them.

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