Opal vs Moonstone: Two Phenomenal Gemstones Compared

Opal and moonstone are both classified as "phenomenal" gemstones, meaning they display special optical effects rather than deriving their beauty from a single body colour. However, the phenomena they display are entirely different, as are their mineral compositions, durability, and price. This guide explains the key differences and helps you choose between these two captivating stones.

Key Differences at a Glance

Understanding the Optical Effects

Play of Colour (Opal)

Opal's play of colour is caused by the diffraction of white light as it passes through the stone's orderly array of microscopic silica spheres. This produces flashes of spectral colour, including reds, oranges, yellows, greens, blues, and violets, that shift and change as the stone moves. The effect is dynamic, vibrant, and unlike anything else in the gemstone world.

Adularescence (Moonstone)

Moonstone's adularescence is a soft, billowy glow that appears to float beneath the surface of the stone, moving as the viewing angle changes. This effect is caused by the scattering of light between alternating layers of orthoclase and albite feldspar within the crystal. The glow is typically white or blue and creates a mystical, ethereal appearance.

The key distinction is that opal displays multiple spectral colours in sharp, defined flashes, while moonstone displays a single, soft, floating glow. Both effects are beautiful, but they appeal to different aesthetic sensibilities.

Colour and Appearance

• Opal – Comes in many body colours: white, black, crystal, and fire. The play of colour superimposed on these body colours creates infinite visual variety. No two opals are alike.
• Moonstone – Most commonly appears as translucent to semi-transparent with a white, grey, or peach body colour topped by a blue or white adularescent glow. Rainbow moonstone (actually labradorite) displays a multi-coloured sheen.

Durability

Both opal and moonstone are relatively soft gemstones, making them less suitable for everyday ring wear compared to sapphire or ruby.

Opal (5.5-6.5 Mohs) is softer and contains water, making it vulnerable to drying, crazing, and thermal shock. Moonstone (6-6.5 Mohs) has similar hardness but has perfect cleavage in two directions, meaning it can split if struck at the right angle. Both stones require protective settings and mindful wearing habits.

For rings, both benefit from bezel settings or designs with protective shoulders. Both are better suited to earrings, pendants, and brooches for regular wear. See our hardness chart for context.

Value and Pricing

The value range for opal is far broader than for moonstone. Fine black opals from Lightning Ridge can command $20,000-50,000+ per carat, placing them among the most expensive gemstones in the world. White opals and Ethiopian opals are much more accessible, ranging from $10 to a few hundred dollars per carat.

Moonstone prices are generally more modest, with most fine material falling between $20 and $200 per carat. The most valuable moonstones are transparent with a vivid blue adularescence, typically from Sri Lanka or Myanmar. Rainbow moonstone (labradorite) is the most affordable variety.

Sources

• Opal – Australia (90% of world supply), Ethiopia, Mexico, Brazil, and Honduras.
• Moonstone – Sri Lanka, India, Myanmar, Madagascar, Tanzania, and the United States.

Treatments

• Opal – May be smoke-treated, polymer-impregnated, or assembled into doublets and triplets. Always ask whether an opal is solid or assembled.
• Moonstone – Generally untreated. Some moonstone may be coated to enhance adularescence, but this is uncommon. The vast majority of moonstone is sold in its natural state.

Jewellery Styles

Both opal and moonstone have a distinctive, romantic quality that makes them popular in bohemian, vintage, and art nouveau jewellery styles. They both work beautifully as cabochon centrepieces in artistic, freeform designs.

• Opal – Makes a dramatic statement with its kaleidoscopic play of colour. Often set in gold to complement its warmth and depth. Boulder opal's irregular shapes inspire unique, one-of-a-kind designs.
• Moonstone – Creates an ethereal, mystical mood. Pairs beautifully with sterling silver, white gold, and rose gold. Particularly popular in stacking rings and layered necklace designs.

Care Comparison

Both stones require gentle care. Clean with a damp cloth or lukewarm soapy water only. Never use ultrasonic or steam cleaners for either stone. Opal has the additional concern of water content: it should not be stored in very dry environments or exposed to extreme temperature changes, as this can cause crazing. Moonstone should be protected from sharp impacts that could trigger cleavage fracture. For detailed instructions, see our jewellery care guide.

When to Choose Opal

• You want a gemstone with dramatic, multi-coloured visual effects
• You appreciate the uniqueness of one-of-a-kind stones
• You are buying an October birthstone
• You are drawn to bold, dynamic jewellery designs
• You are willing to invest in proper care for a delicate stone

When to Choose Moonstone

• You prefer a soft, ethereal glow over flashy colour play
• You want a mystical, romantic aesthetic
• You are buying a June birthstone alternative
• You prefer a more affordable option
• You want an untreated, natural gemstone

Conclusion

Opal and moonstone are both enchanting gemstones that captivate through their unique optical phenomena. Opal dazzles with its kaleidoscopic play of colour, offering endless variety and drama. Moonstone beguiles with its soft, mystical adularescence, evoking moonlight and romance. Both require gentle care and protective settings, but both reward their owners with a beauty that conventional coloured gemstones simply cannot replicate.