October Birthstone: Opal - A Kaleidoscope of Colours
October has two birthstones — opal and tourmaline — but opal is the older and more iconic of the pair. Named from the Sanskrit upala (meaning precious stone), it has been prized for its play of colour since at least the time of Ancient Rome. This guide covers what opal is, how it forms, the main varieties, and what to look for if you are buying one.
In short: Opal is October birthstone, prized for its play-of-colour — flashing reds, blues and greens from light diffraction through silica spheres. Australian black opal is rarest, with top stones reaching £1,500-£5,000 per carat; white opal starts at £30-£150 per carat. Mohs 5.5-6.5 means opal is softer than diamond, so favour bezel settings and pendants over daily-wear rings. Each stone colour pattern is unique.
What gives opal its play of colour
Opal is a hydrated silica gel that hardens into structured spheres of silicon dioxide. When those spheres are uniform in size and packed in an orderly grid, they diffract light into spectral flashes — the famous play of colour that no other gemstone produces. The size of the spheres determines which colours appear: smaller spheres produce blues and violets; larger ones produce reds and oranges. A precious opal with a full spectrum across its surface is exceptionally rare and valuable.
The main types of opal
Australian black opal (mined primarily at Lightning Ridge in New South Wales) has a dark body tone that makes its play of colour appear especially vivid. White opal, also from Australia, has a milky body and softer colour play. Boulder opal forms in ironstone matrix and is often cut with the host rock left attached. Mexican fire opal is transparent to translucent with red, orange, or yellow body colour — sometimes with no play of colour at all, prized for the body hue alone. Ethiopian Welo opal, discovered in 2008, has become a major source of bright, affordable opal with intense colour play.
