Coloured stones
Siam rubies — how to tell what you are actually buying
Chanthaburi is one of the world's coloured-stone capitals. It is also where heated, glass-filled and synthetic rubies sit side by side. Here is how to tell them apart.
Our bench is in Rayong, a little over an hour from Chanthaburi — which happens to be one of the largest coloured-stone trading and cutting centres on earth. A great many of the rubies and sapphires sold in Hong Kong or London passed through cutters in this province first.
The advantage is that we buy close to the source. The disadvantage is that treated and synthetic material is equally close.
Colour drives the price
Ruby is red corundum. Below a certain saturation the same mineral is classified as pink sapphire instead, and the price difference is substantial.
The most valued colour is called pigeon blood — a deep red with a faint blue-pink undertone, not so dark it goes black, not so bright it reads orange. Burmese material often reaches this. Siam rubies from Chanthaburi tend toward a deeper, more sober oxblood red.
What buyers overlook is brightness. A very dark stone with no life is worth less than a slightly lighter one that glows. Always look in daylight. Display case lighting flatters everything.
Heat treatment, and the thing that is not heat treatment
Over 95% of rubies on the market are heated. This is normal, permanent and accepted worldwide — controlled high temperature dissolves inclusions and evens out colour.
What is not normal is lead-glass filling. Badly fractured, low-grade material is infused with molten glass so the cracks disappear and the stone looks clean.
| Type | Acceptable | Relative price | What to know |
|---|---|---|---|
| Unheated | Highest | Very expensive | Rare, must be stated on a report |
| Traditionally heated | Universally accepted | Market standard | Permanent and stable |
| Lead-glass filled | Must be disclosed | A fraction of the above | Glass can dissolve in cleaning solutions |
| Synthetic | Must be disclosed | Cheapest | Lab-grown, chemically identical |
Glass-filled rubies can cost a tenth of properly heated stones while looking similar to the naked eye. Worse, a jeweller's torch or an acid cleaning bath will destroy them.
The only reliable answer is an independent report from a laboratory such as GIT, which states the treatment explicitly.
Clarity and cut
Almost every natural ruby has inclusions. Unlike diamonds, this is expected and does not hurt value much. Fine needle inclusions called silk, when evenly distributed, actually soften and spread the glow.
What to avoid are fractures that break the surface, since those are structural weak points.
Coloured stones are not cut to fixed proportions the way diamonds are. The cutter works to hold weight and show colour at its best. What to check is the depth of the pavilion — a very deep belly means you are paying carat weight for material buried out of sight while the face-up size stays small.
Siam or Burmese ruby — which is better
It is more a difference of hue than of quality. Burmese material tends toward a bright pinkish red the international market prices highest. Siam rubies are deeper and more sober, which many buyers prefer, and they are more affordable at the same size.
Should I buy loose stones at the Chanthaburi market myself
Prices are genuinely lower if you know what you are doing. But there are no price tags and no reports at the market, and anyone who cannot separate a heated stone from a glass-filled one may well pay more than they would at a documented shop. If you go, buy conditionally and have the stone tested at GIT before final payment.
Can a ruby be worn every day
Yes. Ruby is 9 on the Mohs scale, second only to diamond, and stands up well to daily wear. The exception is lead-glass-filled material, which should come off before washing up or swimming.