Blue Topaz

Natural blue topaz exists and is pale, uncommon and expensive. Essentially every blue topaz in ordinary jewellery was colourless when it came out of the ground.

Is blue topaz natural or treated?

Colourless topaz is irradiated to create colour centres in the crystal, then heated to stabilise the result. The colour is permanent, goes right through the stone, and will not fade in normal wear. Treated material is held for a period until residual radioactivity has decayed to safe levels before it is released for sale — a regulated process in the countries that do it.

The three trade names describe intensity, not origin. Sky blue is the palest. Swiss blue is the bright, saturated mid-tone most people picture. London blue is the deep, slightly grey-inky end.

Treated topaz is real topaz. The important thing is that it is described honestly and priced as what it is, which is an abundant, beautiful, thoroughly modern stone rather than a rarity.

What should you look for in blue topaz?

Topaz has good clarity as a rule, so visible inclusions are a reason to pass. Judge colour evenness and cut. Topaz's refractive index is moderate, so a well-proportioned stone is lively and a shallow one windows badly — hold it over print and look through the table.

Is blue topaz durable enough for daily wear?

Mohs 8, which sounds excellent, and in terms of scratch resistance it is. But topaz has perfect basal cleavage: a single plane along which it will split cleanly if struck at the wrong angle. Hardness and toughness are different properties, and topaz is a good demonstration of the difference.

In practice: it will stay bright and unscratched for decades, and it does not enjoy being knocked hard on an edge. Setters know to avoid excess pressure on the girdle for the same reason.