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Composite gemstones: Lead glass filled rubies

Updated: Jun 16, 2021

If you read about my field gemmology trip with Vincent Pardieu in April 2017, you’ll know how I learned about the existence of composite gemstones and lead glass filled rubies.

Lead glass filled rubies entered the market in 2004, but since many people (especially hobbyists and tourist collectors) are still unaware of their presence, I wrote about them in ‘Buying Gemstones and Jewellery in Thailand.’ After all, Thailand is a world hub for gemstone treatment and continues to be where most of the lead glass filled rubies (also known as ‘composite rubies’) are manufactured these days.

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So, for those of you who want to know more about lead glass filled rubies, here’s an excerpt from the book:


In the grand scheme of things, 95% of all gemstones sold have been treated in some way. With rubies and sapphires, the figure is closer to 99%. Heat treatment is the norm and something that you shouldn’t usually worry about — unless you are intending to buy a natural, unheated gemstone, in which case you would be investing thousands of pounds and would expect it to come with a lab report. Be on your guard if someone tries to sell you a good-looking, ‘untreated’ stone!

Gemstone Detective |

Remember: if it looks too good to be true, it almost certainly is.

Thailand is considered a world leader in gemstone treatment techniques, which can work both to the buyer’s advantage or against it.

Heat treatment simply enhances a gemstone’s natural beauty and clarity. Thailand is famous for the skill of its craftspeople, who have over the past 30 years or so developed sophisticated variations on the traditional art of heating gemstones to improve their colour.

Unless you’re looking to spend a vast amount of money, your primary aim should be to make sure the gemstone you are about to buy is genuine and not a piece of cheap fakery.

This said, there is a huge difference between rubies and sapphires that have been fissure treated in the traditional way and ‘composite’ gemstones, which are formed in the lab using a fairly recent technique. The appearance of traditionally fissure-treated rubies and sapphires is enhanced by using miniscule amounts of glass to fill any tiny inclusions on the surface of the stone. Composite gems are formed from lower quality corundum, which is acid treated to remove all the inclusions. The holes left behind are then filled with leaded glass. Leaded glass is very clear, but is even softer than ordinary glass and this means that composite gems are significantly less tough than their traditionally fissure-treated cousins.

Not only is the leaded glass in composite gems less tough than ordinary glass, its presence makes analysis of the corundum itself much more difficult. This means that it is harder for a grading laboratory to determine the quality and weight of the original stone. Some composite stones are more than 40% glass! Furthermore, a composite ruby or sapphire whose corundum has many or deep fractures is difficult to distinguish from one whose corundum has only minor fractures. The former is much less durable than the latter.

Composite gemstones lead glass rubies |


Buying a composite gem is just fine if you know what you’re buying and have paid an appropriate price for it. The problems come when dealers knowingly — or unknowingly — try to sell you a composite ruby as a traditionally fissure-treated stone. Not only will you be paying much more than the stone is worth, but your piece of jewellery will be far more susceptible to damage and degradation.

If you decide to get the gem re-set or professionally cleaned, if you need a ring re-sized, or even if you wear your jewellery while cleaning or cooking, a composite ruby is likely to be damaged irreparably.


Composite gemstones are easy to identify if you can use a jeweller’s loupe — a small magnifying eyepiece used to examine gems. At 10x magnification under a strong light you should be able to see gas bubbles in the glass, heavy fracturing and an effect that looks like streaks of a different colour flashing across the stone.

Want to learn more about gemstones? Then join our tribe and receive a FREE Ebook all about Birthstones. It's a great starting point if you're just starting out in the gem trade.

Kim Rix GG GIA

Be sure | Be smart | Buy with confidence

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