Natural glass tends to be overlooked as a gemstone because of the extensive use of manufactured glass in gemstone imitations. However, both obsidian and moldavite have interest value – the former for its patterns and optical effects and the latter for its origin in outer space.

Types of natural glass

Obsidian is formed when molten lava cools too rapidly for crystallization to occur. It is semitranslucent to opaque and found in black, smoky brown, grey, blue, red and green. Blue and green are less common colours, and red is the rarest. The colour can be uniform or patterned, and can have twisted or straight colour bands caused by the solidification of flowing lava. It may also contain crystal leaflets or tiny gas bubbles that sparkle as they catch the light. Obsidian can be strongly iridescent, showing purple and green patterns as light reflects off its internal structure. Snowflake obsidian is well known for the white marks that resemble snowflakes, which are caused by internal bubbles or crystallites.

Moldavite, or tektite, was first discovered at the end of the 18th century in Czechoslovakia. It is thought that the material either originates from meteorites that melted as they passed through the earth’s atmosphere or is a result of rock shock melting as a large meteorite or comet impacted on the ground. The colour of moldavite is light to dark green, dark brown, grey or black. It can occur as button-shaped pieces with a rough black to brown surface covered in cooling cracks, or as green translucent to transparent material with a distinctive craggy surface.

Treatments and imitations

  • It is impossible to distinguish between natural glass and synthetic glass, as any visual characteristic of the natural material can be readily manufactured. However, synthetic glass is easier to melt.
  • Moldavite could be confused with green diopside or tourmaline, but it lacks their brilliance and dispersion. It is sometimes called bohemian or water chrysolite, but this is a misnomer as the name chrysolite is usually associated with peridot (olivine).

Pricing natural glass
Obsidian is in abundant supply and, except for outstanding mineral specimens, is inexpensive. Moldavite is not particularly rare, but is valued by collectors because of its unusual origin.

Working with natural glass

  • Obsidian and moldavite are fairly soft gemstones and both can be easily scratched and cracked.
  • Obsidian has been used in jewelry for many years and can be found in bead and cabochon form. Clear green moldavite is suitable for jewelry, yet it rarely appears in designs; the green colour is not that attractive and the glass material lacks the sparkle normally found in gemstones. Moldavite, especially the transparent to translucent green material, is more appealing when left in its natural state, and designers have chosen to work with it in that form rather than as a faceted stone.