Glass has been used in jewelry for thousands of years. This article deals exclusively with the commercially or artisanally created variety of this material that is used to simulate gemstones. Obsidian, natural volcanic glass, is described in its own entry. Other varieties of natural glass are described in a separate entry.
All glass has low value. All sold by piece, rather than by carat.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Varieties||Cristinite, Crystal Glass, Lead Glass, Goldstone, Laserblue, Libyan Desert Glass, Natural Glass, Macusanite, Obsidian, Apache Tears, Fire Obsidian, Mahogany Obsidian, Rainbow Obsidian, Sheen Obsidian, Snowflake Obsidian, Paste|
|Refractive Index||1.470 - 1.700|
|Colors||All and colorless|
|Fracture||Conchoidal, inclusions may cause splintery or granular fractures|
|Hardness||5 - 6|
|Specific Gravity||2.30 - 4.50|
|Dispersion||.009 - .098|
|Enhancements||Coatings, common, easily scratched|
|Transparency||Transparent to Opaque|
|UV Long||Not diagnostic|
|UV Short||Not diagnostic|
|Absorption Spectrum||Not diagnostic|
|Phenomena||Adventurescence, adularescent, asterism, chatoyant, color change, iridescent, orient (by adding coatings), play of color (simulates opal but different mechanism).|
|Identifying Characteristics||Singly refractive (ADR common,) bubbles, swirl marks, warm to touch, mold marks, orange peal effect, concave facets, rounded fact edges.|
|Formula||Primarily silica (SiO2) with many additives|
|Etymology||From the Proto-German glasam|
Glass is essentially silicon dioxide (SiO2). Quartz sand is the primary source. By itself, glass is unimpressive with very little brilliance, color, or dispersion. Color is achieved by the addition of metals and minerals. Lead is frequently added to raise the refractive index and dispersion. It also makes the material less brittle, which is a feature of lead crystal glass wear. (Of course, crystal is an inappropriate adjective for an amorphous material). In gemology, we find lead not only in simulated gems but also in the hemispheres of our refractometers. It is also the addition of lead that gives some glass such a high specific gravity.
The back sides of faceted glass stones are often covered with foil, which adds both color and brilliance. Foil backs are usually called rhinestones. They are easily distinguished by looking at the pavilion of the stone. They are opaque and usually a different color than you see face up. Today, our costume jewelry is still filled with richly colored rhinestones.
Glass can be treated to imitate a wide number of gemstones and phenomena. One of the most popular treatments is the creation of goldstones. Copper shavings are added to create a glittering appearance. These pieces look like nothing in the natural world and have become very popular.
Glass is usually easy to distinguish with a loupe. Its primary inclusions are air bubbles and swirl marks. What appear to be faceted gems are usually molded. The material shrinks slightly as it cools, leaving concave facets. Glass also conducts heat much faster than crystalline materials, so it feels warm to the touch.
- Alexandrium, color change glass
- Aurora Borealis, glass with iridescent coating. This process was created by the Swarovski corporation in 1956. (Not to be confused with natural obsidian of the same name).
- Cat’s eye and fire eye, chatoyant glass
- Chatons, colorless glass with foil back
- Fiance, colored glaze on base of pottery
- Goldstone, contains copper or gold colored inclusions causing adventurescence
- Meta Jade, imitation jade also called Kinga Stone or Victoria Stone
- Paste, common imitation gem
- Rhinestone, colorless glass with foil back
- Strass, high lead content glass
- Slokum Stone, imitation opal