Glass is the oldest and most common gem imitation. It can be made in any color, cut into any shape and molded cheaply, so it has stood in for rubies, emeralds, sapphires and diamonds since ancient times. Glass jewelry also has a respected history of its own: Georgian paste, Victorian costume pieces and 20th-century rhinestone jewelry all have collectors. Knowing the signs of glass helps you judge what a stone really is.
What glass is
Glass is an amorphous solid. Its atoms have no regular crystal structure, which makes it different from almost every natural gem. Most glass is based on silica (silicon dioxide), melted with other ingredients that lower the melting point and change its properties. Common additives include soda, potash, lime, lead oxide and boron.
Because it has no crystal structure, glass is singly refractive. It has no cleavage and breaks with a smooth, shell-like (conchoidal) fracture. Its hardness is usually about 5 to 6 on the Mohs scale, lower than quartz. The page on crystal systems in gemstones explains how crystal structure shapes gem optics.
Typical properties of glass used in jewelry:
| Property | Typical range |
|---|---|
| Refractive index | About 1.50 to 1.70 (higher in high-lead glass) |
| Specific gravity | About 2.3 to 4.5, depending on lead content |
| Mohs hardness | About 5 to 6 |
| Optic character | Singly refractive (isotropic) |
| Fracture | Conchoidal |
Natural glasses also exist. Obsidian forms when lava cools too fast to crystallize, and moldavite is a tektite, a glass linked to a meteorite impact. Those are covered on the page about meteorite in jewelry. This page deals with man-made glass.
Glass as an imitation through history
Ancient Egyptians made glass beads and inlays to imitate turquoise, lapis lazuli and carnelian. Egyptian faience, a glazed quartz ceramic, served a similar purpose. The page on ancient Egyptian jewelry covers these materials. Roman glassmakers produced colored glass imitations of emerald and other gems, and Roman writers already complained about fake gems made of glass.
In medieval and Renaissance Europe, Venice and its glassmaking island of Murano became famous for colored glass. Venetian glassmakers also made goldstone, a glass full of copper crystals that gave its name to the natural stone aventurine.
Paste jewelry
"Paste" is the traditional name for glass cut and set as a gem in fine jewelry. The term probably comes from the soft, paste-like state of the melted ingredients.
Strass and lead glass
In the 1730s, the Paris jeweler Georges Frédéric Strass became known for colorless glass imitations of diamond. His glass contained a high proportion of lead oxide, which raises the refractive index and dispersion, so the stones sparkle more than ordinary glass. In some European languages "strass" still means rhinestone or paste. Similar high-lead glass is also called flint glass or lead crystal.
Georgian paste
Paste reached its height in the 18th and early 19th centuries, roughly the Georgian era in Britain. Paste pieces were made by skilled jewelers in silver and gold, often with the same workmanship as diamond jewelry. Fashionable people who owned diamonds also wore paste openly.
Georgian paste stones were usually set in closed-back settings. The jeweler placed a foil, a thin sheet of metal (often silver or colored foil), behind each stone to reflect light back through it and add color or brilliance. Some stones had a black dot painted on the culet to imitate the open culet of old diamond cuts. Moisture getting into closed settings can tarnish the foil, so old paste sometimes looks dull or patchy. Antique paste is collected today for its history and workmanship.
Rhinestones
The name "rhinestone" first referred to rock crystal pebbles collected from the Rhine River in Europe. Later it came to mean colorless glass imitations of diamond, and then colored ones too.
In the 1890s, Daniel Swarovski patented a machine for cutting glass stones with precision. His company, founded in Wattens, Austria, made high-lead crystal stones that became a staple of costume jewelry, fashion and stage costume. By the mid 20th century, rhinestone jewelry was everywhere, from Hollywood film costumes to department store brooches. Many rhinestones have a mirror-like metallic coating on the back (foil backing) to increase their sparkle.
Victorian costume pieces and later 20th-century costume jewelry made heavy use of glass. Our page on Victorian jewelry covers that period's materials.
Other glass in jewelry
- Molded glass cabochons and beads in every color.
- Glass with a coating that creates an iridescent "aurora borealis" effect, introduced in the 1950s.
- Opalescent and slag glass imitating moonstone, opal and other phenomenal gems.
- Glass with parallel fibers (fiber optic glass) imitating cat's eye stones.
- Lead glass used to fill fractures in rubies, creating composite stones discussed on ruby treatments.
- Glass in doublets, where a thin garnet top is fused to a glass base. These garnet-topped doublets were common imitations in the 19th and 20th centuries.
How to spot glass
No single home test proves a stone is glass, but several signs together point that way. A 10x loupe helps a great deal.
Gas bubbles
Round or elongated gas bubbles are the classic sign of glass. They are trapped air or gas from the melt. Natural gems rarely contain perfectly round bubbles, though some contain liquid inclusions with a gas bubble inside.
Swirl marks
Glass often shows curved flow lines or swirl marks, where the melt was stirred or poured. These look like heat shimmer on a hot road. Natural crystals have straight growth lines or angular color zoning.
Molded edges and soft facet junctions
Mass-produced glass stones are often pressed in molds. Molded stones have rounded, soft facet edges instead of the sharp junctions of polished gems. The facet surfaces may be slightly concave, and you may see a mold seam around the girdle. Look for orange-peel texture on the facets, too.
Wear and chips
Glass is softer than most gems. Old glass stones often show scratched tables and chipped facet edges, especially on corners. A stone that claims to be sapphire but has worn, rounded facet edges deserves a closer look.
Warmth to the touch
Glass conducts heat more slowly than most crystals. A stone that warms up quickly on your lip or in your hand may be glass, while quartz, topaz and corundum feel cooler for longer. This is a rough indicator only.
Single refraction
Glass is singly refractive. A gemologist with a polariscope or refractometer can confirm this and measure the refractive index. Glass may show patchy strain patterns (anomalous extinction) in a polariscope. These tools are described on gemological instruments, and our guide on how to identify gemstones covers the wider process.
Caring for glass jewelry
Clean glass stones gently with a damp cloth and mild soap. Do not soak foil-backed or closed-back pieces, since water can get behind the stones and damage the foil. Avoid ultrasonic cleaners, which can loosen glued stones in costume jewelry. Store glass pieces separately to avoid scratches.
Frequently asked questions
What is paste jewelry?
Paste is glass cut and set as a gemstone. Georgian paste, made in the 1700s and early 1800s, was often high-lead glass set in silver or gold with foil backing.
Are rhinestones glass?
Most rhinestones are glass, often high-lead crystal glass, sometimes with a reflective coating on the back. Some modern rhinestones are acrylic or other plastics.
How can you tell if a gem is glass?
Look for round gas bubbles, swirl marks, rounded molded facet edges and worn or chipped facets. A gemologist can confirm with a refractometer and polariscope.
Is antique paste jewelry worth anything?
Good Georgian and Victorian paste in original condition is collected for its age and workmanship. Value depends on condition, quality of the setting and design.