Glass
Mahogany Obsidian
Indonesia
69.01 carats
© gemselect.com
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.
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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:28.24 cts
Total photos:43
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| 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. |
| Glass Treatments | |
| Can be coated with metal foils - Gemmological Tables, Ulrich Henn and Claudio C. Milisenda, 2004, p 2 | |
| Physical Properties of Glass | |
| Mohs Hardness | 4 to 6, Gemmological Tables (2004) More from other references |
| Specific Gravity | 2.39 to 4.00, Gemmological Tables (2004) More from other references |
| Fracture | Conchoidal, Management Team (2012) |
| Optical Properties of Glass | |
| Refractive Index | 1.48 to 1.70, Gemmological Tables (2004) More from other references |
| Optical Character | Isotropic, Gemmological Tables (2004) |
| Colour | |
| Colour (General) | Various, play of colour, Gemmological Tables (2004) More from other references |
| Causes of Colour | Yellowish green, Fe2+ in octahedral coordination. Brown Fe3+ in octahedral coordination, Pragmatic Spectroscopy For Gemologists (2011) |
| Transparency | Transparent,Translucent,Opaque, Gemmological Tables (2004) More from other references |
| Lustre | Vitreous, Management Team (2012) |
| Fluorescence & other light emissions | |
| Fluorescence (Short Wave UV) | Strong chalky fluorescence common, Blue Chart Gem Identification (2010) |
| Crystallography of Glass | |
| Crystal System | Amorphous, 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 |