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Precious stones
Semi-precious stones
Technical information
  Forbidden terms
  Optical properties
  Mechanical properties
  Crystal systems
History of the stones
Optical properties

Color :
It is strongly related to the way in which it absorbs the light. Certain colors of spectrum (of the rainbow) are absorbed and the others are reflected. It's the reflexion that give colouring. The gems are classified in foor categories :
  • allochromatic gems: colouring is due to traces or impurities which do not form part of the principal chemical components of the gem (example: Ruby and Sapphire),
  • idiochromatic gems: colouring is due to essential chemical components of the gem, these gems will always have a narrow range of colour (example: Peridot which is always green),
  • blend gems: colouring is multiple (bicolour, tricolour,…) and it is distributed either unequally, or by zone in relation to the growth (example: Tourmaline “watermelon”),
  • pleochroic gems : these gems pésentent several nuances (dichroism: 2, trichroism: 3,…) according to the angle of sight (example: Cordierite)

Refractive index :
The light which is not reflected and which thus penetrates in the stone is slowed down and deviated (refracted): importance of this deviation give the refractive index (RI).
For the stones which have a double refraction, we measure another parameter called Birefringence (B).
The refractive index (and if there is birefringence) makes it possible to identify a stone.

Brilliance :
It determined by the way in which surface of a stone reflected the light.
Shining brilliance : the light is reflected like on a mirrorr.
Dull brilliance: the light is very poorly reflected.
Adamantine brilliance: comparable with that of diamond.
Vitreous brilliance: the light is poorly reflected, low transparency.

Interference :
It is due to the reflexion of the light by the internal structures of the stone. Certain stones thus produce all the colors of the spectrum as for the Opal (iridescence) or of the effects of gleaming as for Moon stone (adularescence).

Chatoyance and Asterism :
For the stones cut out of cabochon, the light reflected by the internal structures (cavities, inclusions) can produce an effect of cat's eye (chatoyance) or star (asterism).
Stone presenting a chatoyance: Chrysoberil Cat's eye.
Stone presenting an asterism: Star sapphire.