Blue topaz is the sky-coloured member of the topaz family, an aluminium silicate that grows in beautifully clean crystals. Here's the honest bit: deep natural blue topaz is very rare. Nearly all the blue topaz in jewellery starts as colourless topaz that is irradiated and then heated, producing the popular sky blue, Swiss blue and London blue shades. This is a safe, permanent and openly disclosed trade practice, and the stone itself is completely genuine topaz. Brazil is the main source, and blue topaz is a birthstone for December.
At 8 on the Mohs scale, blue topaz shrugs off scratches, but there is a catch. Topaz has perfect cleavage, meaning a sharp knock in the wrong direction can split it. So while it is fine for regular wear, protective settings help for rings, and it should never go in an ultrasonic cleaner. It is usually faceted, and because large clean crystals are common, you can get a big, bright stone for very little. Warm, soapy water is all it needs.
Aquamarine is its classic look-alike, and a transparent blue apatite has a similar look.
