In his daily routine the average jeweller is sure to encounter glass in many forms. These range from the cheap moulded pastes that are mounted in costume jewellery or massed produced 9ct settings to the rare natural Moldavites that may be encountered set in fine antique pieces.
The natural glasses to be found in jewellery items include:
This is a volcanic glass that often occurs in relatively substantial flows, generally black or dark brown in colour. Uniform jet-black “Apache Tears”, “Snowflake obsidian” that has a black body colour with numerous snowflake-like inclusions and “Mahogany Obsidian” of a warm reddish-brown hue, are the three most popular varieties to be used in jewellery.
These are objects from outer-space that are generally black and opaque but may exhibit interesting shapes.
On the other hand, often inaccurately described as being of extra-terrestrial origin, are black to green impact glasses that were formed as the result of a meteorite striking and fusing earthly bodies of silica. Primarily this took place in the Ries Crater of southern Germany some 14.7 million years ago. Facetted Moldavite was very fashionable in the latter part of the nineteenth century and it was often set together with Bohemian chrome-pyrope garnets and fresh-water pearls.
are glassy objects that can exhibit very interesting shapes. They are the result of powerful forked lightning striking desert sands and they are most frequently encountered in the Sahara Desert.
From left to right: Artificial Glass, Glass, Conchoidal fracture
Artificial Glasses to be found in jewellery items include:
This glass can withstand very high temperatures and it is resistant to thermal shock.
Soda-lime silica glass
Silica, Sodium oxide, lime, magnesia, alumina
The commonest form of glass, soda-lime glass accounts for approximately 90% of all manufactured glass. It is used for windowpanes, bottles, glasses, etc.
Sodium borosilicate glass
Silica, boron-trioxide, soda, alumina
Borosilicate glass is stable at high temperatures and resists thermal shock. It is mainly used for oven-ware.
Silica, Lead oxide, potassium oxide, soda, zinc, alumina
Lead oxide glasses are heavy, bright and highly dispersive. Known as crystal ware they are used for decanters, drinking glasses, decorative table-ware, bowls and vases. They are also employed as a gem simulant material where they are known as Strasse or jeweller’s paste.