Glass

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China
32.25 carats

Glass is an amorphous form of silica (with various percentages of additives) that has been used as a gemstone for millennia.

Clear glass for gems is man-made, formed by fusing silica sand with various additives at temperatures of 1000+°C.

Natural glasses include obsidian, which is is a rapidly quenched lava, and closer to pure silica glasses, such as libyan desert glass.

A unique man-made glass, although not often used in gems for obious reasons, is trinitite, a mildly radioactive glass formed from the fusing of sand by the detonation of the first atomic bomb in 1945.

Common glass has poor dispersion and refractive index, however it is very cheap to manufacture and is known in jewellery terms as paste - often backed with metal foils to increase brilliance.

Adding lead oxide or other additives to glass creates lead glass, or crystal glass, which have much stronger RI and dispersion.

Glass Gemstones by Colour

This table shows the variety of hues this gemstone can be found in. Click on a photo for more information.
 
 
 
 
 

Glass Gemstones by Size

This table shows distribution of Glass gemstone sizes that are listed on this site. This can give a good indication as to the general availability of this gemstone in different sizes.
Contributed photos
Lightest:0.16 cts
Heaviest:168.6 cts
Average:28.74 cts
Total photos:42
Do you have a larger Glass? Why not upload a photo?
0.16ct to 17.00ct17.00ct to 33.85ct33.85ct to 50.69ct50.69ct to 67.54ct67.54ct to 84.38ct84.38ct to 101.22ct101.22ct to 118.07ct118.07ct to 134.91ct134.91ct to 151.76ct151.76ct to 168.60ct
General Information
Varieties/Types:
Cristinite - A trade name for an artificial glass gem material produced in Australia.
Crystal Glass - A glass containing additives to increase dispersion and RI.
Lead Glass - A crystal glass containing up to 40% wt. lead oxide.
Goldstone - A glass containing flecks of copper.
Libyan Desert Glass - A natural yellow glass found in the western desert of Egypt.
Apache Tears - Small rounded pebbles of black Obsidian found in American Southwest.
Mahogany Obsidian - A variety of Obsidian with black and red banding.
Rainbow Obsidian - Obsidian with multicolored iridescence.
Sheen Obsidian - A variety of obsidian exhibiting a golden sheen effect.
Snowflake Obsidian - A variety of obsidian containing white "snowflake" crystal patterns of the mineral cristobalite.
Paste - Glass without additives, also known as common glass.
Photos of natural/un-cut material from mindat.org
Glass Treatments
Can be coated with metal foils - Gemmological Tables, Ulrich Henn and Claudio C. Milisenda, 2004, p 2
Physical Properties of Glass
Mohs Hardness4 to 6
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
Specific Gravity2.39 to 4.00
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
FractureConchoidal
Gemdat.org, Management Team (2012)
Optical Properties of Glass
Refractive Index1.48 to 1.70
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
Optical CharacterIsotropic
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
Colour
Colour (General)Various, play of colour
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
Causes of ColourYellowish green, Fe2+ in octahedral coordination. Brown Fe3+ in octahedral coordination
W. William Hanneman, Pragmatic Spectroscopy For Gemologists (2011)
TransparencyTransparent,Translucent,Opaque
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
LustreVitreous
Gemdat.org, Management Team (2012)
Fluorescence & other light emissions
Fluorescence (Short Wave UV)Strong chalky fluorescence common
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
Crystallography of Glass
Crystal SystemAmorphous
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
Inclusions in Glass
Gas bubbles, swirl marks, partly tabby extinction - Gemmological Tables, Ulrich Henn and Claudio C. Milisenda, 2004, p 2
Further Information
Mineral information:Glass information at mindat.org
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