Opal Guide: Play-of-Colour, Types & Buying Considerations

Opal is the October birthstone (see our birthstones guide) and one of the few gemstones whose defining feature is a genuine optical phenomenon rather than colour or clarity alone.

What causes play-of-colour

Opal forms from silica deposited in tiny, uniformly-sized spheres. When those spheres are regularly stacked, they diffract white light into its spectral components as it passes through — producing the flashes of colour that shift as the stone or the viewer moves, called play-of-colour. Irregular or poorly-ordered silica spheres produce common opal with no play-of-colour at all — the internal structure, not the chemistry, is what separates a valuable precious opal from an ordinary one.

The main types

• Black opal — a dark body tone (rated N1-N4 on the trade's darkness scale) that makes any play-of-colour read as more vivid and intense by contrast. Generally considered the finest and most valuable opal type, and predominantly sourced from Lightning Ridge, Australia.
• White opal — a lighter, milky background with gentler, softer play-of-colour. More accessible in price than black opal without necessarily being lower quality — the colour play is real, just set against a paler stage.
• Boulder opal — a thin layer of precious opal naturally attached to its ironstone host rock, left in place rather than cut away. That host rock makes boulder opal significantly tougher and more resistant to cracking than solid opal of comparable thinness, a genuine durability advantage alongside its distinctive dark, mottled backing.
• Fire opal — mostly Mexican in origin, defined by a vivid orange, yellow or red body colour rather than by play-of-colour, which some fire opals show and others don't at all.

Durability is the single biggest buying consideration

Opal sits at roughly 5.5-6.5 on the Mohs scale and, more importantly, contains up to 20% water within its silica structure — which makes it genuinely more prone to cracking (crazing) from heat, dryness or sudden temperature change than almost any other common gemstone. Boulder opal and white opal are the sturdier everyday-wear choices; a fine black opal is better suited to occasional wear or a protective bezel setting than a ring worn and knocked about daily. Never clean opal ultrasonically or with heat — see our jewellery care guide for the safe method.

What drives price

Darker body tone, brighter and more evenly distributed play-of-colour, and a broader spread of spectral colours (red flash is generally the rarest and most valued, more so than blue or green) all drive value upward. Buy opal in person where possible, or from a dealer with a strong returns policy — photography struggles to capture play-of-colour accurately, and the angle-dependent flash that makes a fine opal special is exactly the thing a flat photo can most easily misrepresent.

Related reading

• Birthstones by month: the complete guide
• Mohs hardness scale: which gemstones are safe for everyday wear
• How to clean and care for gemstone jewellery

Source: opal formation, type classification and durability guidance per GIA's Opal Quality Factors and Australian Opal Direct.