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Asterism (gemmology)

 
Asterism (gemmology)

 
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Asterism (gemmology)
 
 
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In gemmology, an asterism is an optical phenomenon
Optical phenomenon

An optical phenomenon is any observable event which results from the interaction of light and matter. See also list of optical topics and optics....
 displayed by some rubies
Ruby

A ruby is a pink to blood-red gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium....
, sapphire
Sapphire

Sapphire refers to gem varieties of the mineral corundum, an aluminium oxide , when it is a color other than red, in which case the gem would instead be a ruby....
s, and other gems
Gemstone

A gemstone or gem, also called a precious or semi-precious stone, is a piece of attractive mineral, which — when cut and polished — is used to make jewellery or other adornments....
 (i.e. star garnet
Star garnet

Star garnets are a special form of the gemstone garnet. While colored like other garnets they display a reflection-like image of a four or six pointed star caused by the inclusion of rutile needles, oriented at 70 and 110 degrees....
, star diopside
Diopside

Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite....
, star spinel
Spinel

The spinels are any of a class of minerals of general formulation A2+B23+oxygen42- which crystallise in the cubic crystal system crystal system, with the oxide anions arranged in a cubic close-packing Bravais lattice and the cations A and B occupying some or all of the octahedral molecul...
, etc) of an enhanced reflective area in the shape of a "star" on the surface of a cabochon
Cabochon

A cabochon or cabachon, from the Middle French caboche , is a gemstone which has been shaped and polished as opposed to facetted. The resulting form is usually a convex top with a flat bottom....
 cut from the stone. Star sapphires and rubies get their asterism from the titanium dioxide
Titanium dioxide

Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula titaniumoxygen2....
 impurities (rutile
Rutile

Rutile is a mineral composed primarily of titanium dioxide, titaniumoxygen2.Rutile is the most common natural form of TiO2....
) present in them.

A distinction can be made between two types of asterism:






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Star Saphire
In gemmology, an asterism is an optical phenomenon
Optical phenomenon

An optical phenomenon is any observable event which results from the interaction of light and matter. See also list of optical topics and optics....
 displayed by some rubies
Ruby

A ruby is a pink to blood-red gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium....
, sapphire
Sapphire

Sapphire refers to gem varieties of the mineral corundum, an aluminium oxide , when it is a color other than red, in which case the gem would instead be a ruby....
s, and other gems
Gemstone

A gemstone or gem, also called a precious or semi-precious stone, is a piece of attractive mineral, which — when cut and polished — is used to make jewellery or other adornments....
 (i.e. star garnet
Star garnet

Star garnets are a special form of the gemstone garnet. While colored like other garnets they display a reflection-like image of a four or six pointed star caused by the inclusion of rutile needles, oriented at 70 and 110 degrees....
, star diopside
Diopside

Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite....
, star spinel
Spinel

The spinels are any of a class of minerals of general formulation A2+B23+oxygen42- which crystallise in the cubic crystal system crystal system, with the oxide anions arranged in a cubic close-packing Bravais lattice and the cations A and B occupying some or all of the octahedral molecul...
, etc) of an enhanced reflective area in the shape of a "star" on the surface of a cabochon
Cabochon

A cabochon or cabachon, from the Middle French caboche , is a gemstone which has been shaped and polished as opposed to facetted. The resulting form is usually a convex top with a flat bottom....
 cut from the stone. Star sapphires and rubies get their asterism from the titanium dioxide
Titanium dioxide

Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula titaniumoxygen2....
 impurities (rutile
Rutile

Rutile is a mineral composed primarily of titanium dioxide, titaniumoxygen2.Rutile is the most common natural form of TiO2....
) present in them.

A distinction can be made between two types of asterism:
  • Epiasterism, such as that seen in sapphire and most other gems, is the result of a reflection of light on parallel arranged inclusions inside the gemstone.
  • Diasterism, such as that seen in rose quartz, is the result of light transmitted through the stone. In order to see this effect, the stone must be illuminated from behind.