The Truth About Treatments Part 1: Colored Gems

Just as wool is dyed, leather is tanned, and wood furniture is stained and sealed, many of the colored gemstones on the market today have been treated to enhance their natural beauty and durability, and to make a wider variety of jewelry-quality gemstones available and affordable.

A treated gemstone is still a real gemstone created by the force of nature. Some of the most common treatments used today, such as heating stones to improve their color, have been used for centuries to finish what nature started. Without heat-treating, there would be no bright blue topaz, the intense blue-violet of tanzanite would be a dull brown, and most rubies and sapphires would have a less-than-vivid hue. Your great-grandmother’s sapphire brooch may well have been heat-treated way back in the 1800s!

Most jewelers do sell treated goods. The practice is ethical as long as treatments are disclosed and their degree of permanence taken into account when pricing the gem. Listed below are the ten treatments that require disclosure by the Federal Trade Commission:

Bleaching: Chemicals such as hydrogen peroxide, diluted acid, or chlorine bleach are used to lighten or remove color or stains. Lotion and skin oils can sometimes stain bleached gems.

Cavity Filling: Fillers such as glass or plastic are used to seal cavities or pits on the surface of a gem. Cavity filling is sometimes used to add weight, too. Heat from a microscope light, a jeweler’s torch, store lighting, or even sunlight can cause fillers to leak.

Colorless Impregnation: Melted wax or plastic is applied to fill the pores of a gem, then allowed to solidify in order to improve the gem’s color and form a protective seal. Heat can destroy the seal.

Dyeing: Chemicals or colored oils are used to add color or to deepen it. Dyes can fade in sunlight.

Fracture Filling: Similar to cavity filling, fracture filling is used to seal narrow breaks in the stone to improve its clarity. Most often colorless glass, plastics, or oils are used as fillers. Heat can cause fillers to leak.

Heat Treatment: Heat is the oldest gem treatment, and the most common. Today, it’s become quite a science: depending on the temperature used and the length and rate of heating and cooling, color can be lightened, darkened, deepened, or changed completely. Heat-treating is very durable, but intense heat from a jeweler’s torch can ruin it.

Irradiation: Scientists began experimenting with gem radiation as early as the 1900s. The process of irradiation rearranges a stone’s atoms and electrons in order to change its color. The new color isn’t always stable, though. Exposure to heat or even daylight is sometimes enough to fade the color.

Lattice Diffusion: By combining extremely high temperatures with chemicals, light-colored gems can be infused with a shallow layer of almost any color. Lattice diffusion can also enhance asterism—the “star” in star ruby or sapphire.

Sugar and Smoke Treatments: These simple surface treatments can darken pale opal and enhance its color display. Whether the stone is soaked in a hot sugar solution or roasted over a fire, the result is the same.

Surface Modifications: Gluing various backings, such as foil, fabrics, or even feathers, onto the undersides of stones can enhance color—or give color where there was none. Coatings such as wax or varnish bring out the luster of porous stones. Painting is also used to improve a gem’s color. A little dab of nail polish on the underside of a gem can give an inexpensive colorless gem the appearance of a ruby.

Here’s a shortlist of the most common treatments for popular gems.
Amethyst: heat treatment
Aquamarine: heat treatment
Chalcedony: dyeing
Citrine: heat treatment
Emerald: fracture filling, dyeing
Jade: impregnation, bleaching and impregnation, dyeing
Lapis Lazuli: dyeing, coating
Opal: impregnation (oil, wax, plastic), sugar treatment, smoke treatment
Pearl: bleaching
Ruby: heat treatment, lattice diffusion, fracture filling, cavity filling
Sapphire: heat treatment, lattice diffusion, fracture filling, cavity filling
Tanzanite: heat treatment
Topaz: heat treatment, irradiation followed by heat treatment
Tourmaline: heat treatment, irradiation
Turquoise: impregnation (wax, plastic, dyes)
Zircon: heat treatment

Some treatments, such as heat and irradiation, are undetectable even by gemologists, so proceed with caution: assume a colored gemstone is treated until proven otherwise.