Most rubies sold today have been treated in some way. Heating is the most common treatment and is stable and widely accepted, while lead-glass filling changes a stone so much that it belongs in a different price category. After color, the treatment a ruby has received is one of the biggest influences on its price.
Why rubies are treated
Natural ruby rough often has purplish or bluish zones, dense silk that clouds the color, or fractures that reach the surface. Treatments aim to improve color, clarity or both. Some, like heating, go back centuries. Others, such as glass filling and beryllium diffusion, appeared around the turn of the 21st century and caught much of the trade off guard until labs worked out how to detect them.
Heat treatment
Modern heating takes place in electric or gas furnaces, often at temperatures well above 1,000 °C. At these temperatures, rutile silk can dissolve back into the corundum, which clears the stone and lets more light through. Heating can also reduce the blue component that makes some rubies look purplish. Material from Mong Hsu in Myanmar, which often has dark bluish centers, is routinely heated for this reason.
Gemologists detect heating mainly with the microscope. Typical signs include:
- Silk that looks broken, dotted or partly dissolved.
- Mineral crystals surrounded by small disc-shaped stress fractures, caused by the crystal expanding faster than the ruby around it.
- Crystals with melted or rounded surfaces.
- Changes in color zoning and, in some stones, changes in the infrared spectrum.
Heating at lower temperatures can leave few traces, so a lab may report that it found no indications of heating rather than stating that a stone was never heated. Heat treatment is permanent. Unheated rubies of comparable quality cost more, and their price gap widens as quality rises.
Flux healing of fractures
When rubies are heated with fluxes such as borax, the flux melts and flows into surface-reaching fractures. It dissolves a little of the corundum along the fracture walls, and as the stone cools, new corundum and glassy flux residue partly seal the break. The result is a cleaner-looking, more stable stone.
Under magnification, the healed areas show glassy, whitish or yellowish residue in networks that resemble fingerprints, sometimes with tiny gas bubbles. Labs usually note the amount of residue on a report, using graded terms such as minor or moderate. Flux-healed rubies hold up to normal wear. A stone with a large amount of residue is worth less than one with little or none.
Lead-glass filling
Lead-glass filled rubies reached the market in large numbers in the early 2000s. The starting material is heavily fractured, low-grade corundum, often nearly opaque. It is cleaned with acid and then heated with a glass rich in lead. The glass has a refractive index close to ruby's, so once it fills the fractures and cavities, the cracks almost disappear and the stone looks transparent. In some pieces, glass makes up a large share of the volume.
Because these stones are part corundum and part glass, labs describe them as "lead glass filled ruby" or "composite ruby," and some in the trade argue they should not be called ruby at all. Signs include:
- A blue or orange flash of color along filled fractures when the stone is tilted.
- Flattened or rounded gas bubbles trapped in the glass.
- Areas of lower luster on the surface where glass reaches the polished face.
- A strong response on X-ray imaging, since lead is dense.
The glass is far less durable than corundum. Mild acids, including lemon juice and the pickling solutions jewelers use after soldering, can etch it. Heat from a jeweler's torch can damage it, and a badly damaged stone can lose much of its transparency. These stones are worth a small fraction of a heated ruby.
Diffusion
Diffusion treatment uses very high heat to push coloring elements into corundum from outside. Beryllium diffusion became widely known in the early 2000s, mostly in sapphire, and has been applied to some rubies to shift their color. Beryllium is a light element that standard lab screening cannot detect, so labs use LA-ICP-MS or similar methods to find it.
Surface diffusion with chromium has also been tried. It creates only a thin colored layer. When such a stone is immersed in a liquid and viewed over diffused light, color concentrates along facet edges and the girdle. Recutting or a deep chip exposes the paler stone underneath.
Dyeing and oiling
Low-grade, fractured rubies are sometimes dyed red. Some are first quench-crackled, heated and plunged into cold liquid to create fractures for the dye to enter. Dye collects in the cracks and shows as red threads or patches under magnification. Solvents can lift some dyes, so dyed stones need careful handling.
Oiling fills surface-reaching fractures with colorless or red oil or resin to hide them. The practice has a long history in the Burmese trade. Oil can dry out or sweat from a stone over time and can be removed by heat, solvents or ultrasonic cleaning.
How treatments are disclosed
In the United States, the Federal Trade Commission's Guides for the Jewelry Industry call for disclosure of a treatment that is not permanent, that creates special care needs, or that has a large effect on value. Trade groups such as AGTA and CIBJO set similar disclosure rules for their members. Lab reports use wording such as "no indications of heating," "indications of heating," "heated, with residues in fissures" and "lead glass filled."
Ask for treatment status in writing on the invoice. For a ruby of any real value, an independent lab report is the most reliable record. The overview of gemstone treatments explains common disclosure terms, and the guide on how to buy a gemstone covers what to ask a seller. For an untreated stone with a strong origin story, see the page on Mogok rubies.
Care differences by treatment
| Treatment | Stable in normal wear | Ultrasonic and steam | Notes |
|---|---|---|---|
| Heat only | Yes | Usually safe | Care is the same as for untreated ruby |
| Flux healed | Yes | Use caution | Avoid repeated high heat |
| Lead-glass filled | No | Avoid | Acids and torch heat damage the glass |
| Dyed | No | Avoid | Solvents can remove dye |
| Oiled | No | Avoid | Oil can dry out or be cleaned away |
| Beryllium diffused | Yes | Usually safe | Needs disclosure and costs far less than heated ruby |
| Surface diffused | Yes | Usually safe | Recutting or chips expose paler material |
Warm soapy water and a soft brush are safe for every ruby on this list. The guide to cleaning jewelry safely has more detail, and the page on gemstone durability explains why fractures change how a stone should be worn.
Frequently asked questions
What percentage of rubies are heated?
No exact figure exists, but the trade and the major labs agree that most rubies on the market have been heated. Unheated stones with lab reports are the exception and sell at a premium.
Is a glass-filled ruby a real ruby?
It contains natural corundum, but glass fills much of the stone and holds it together. Labs call it a composite or lead glass filled ruby, and it should be sold under that description at a much lower price.
Can a jeweler repair a ring set with a glass-filled ruby?
Yes, with care. The jeweler should remove the stone before using a torch or pickling solution, because heat and acid can damage the glass filler.
Does heat treatment reduce a ruby's value?
Heated rubies are accepted and sold widely, but an unheated ruby of the same color, clarity and size costs more. The gap is largest for fine, large stones.