Iris quartz, sometimes called rainbow quartz or anandalite in certain markets, is rock crystal or smoky quartz that displays internal flashes of spectral colour when light strikes healed fractures within the stone. The effect arises not from any pigmenting element but from thin-film interference: a sub-microscopic gap or refractive index discontinuity along a healed crack acts as a thin film, splitting reflected light into its component wavelengths in much the way that an oil film on water or a soap bubble does.

Mechanism

When quartz crystals fracture during growth or after burial and are subsequently sealed by further silica deposition, the rejoined surface often retains a discontinuity on the order of a few hundred nanometres. Light entering the stone reflects from both faces of this near-invisible film, and the two reflections interfere to produce iridescence whose colour depends on the film thickness and the viewing angle. The same mechanism produces the colour on a butterfly's wing or a film of petroleum.

Because the iridescence depends on planar features within the stone, iris quartz typically shows ribbons or sheets of colour that wink in and out as the stone is rotated. The rest of the crystal remains transparent and colourless, or in the case of smoky iris quartz, transparent grey-brown.

Natural and induced varieties

Naturally occurring iris quartz is uncommon. The bulk of material on the market today is produced by a controlled treatment in which pre-fractured quartz is heated and quenched to widen the internal cracks, then occasionally vapour-coated with metallic oxides to deepen the colour play. The treated product is sometimes sold as rainbow quartz, fire and ice quartz, or under proprietary trade names. Disclosure varies, and serious buyers will ask whether the iridescence is natural or treatment-induced, since the two command very different prices.

The trade name anandalite has been used for a Himalayan-sourced iris quartz marketed largely through metaphysical channels; gemmological literature treats this as a natural iris-effect quartz from the Indian subcontinent rather than a distinct species.

Cutting and use

Iris quartz is generally cut as cabochons, polished tumbled pieces, or freeform points where the iridescent planes can be oriented towards the viewer. Faceting is uncommon because the brilliance of a faceted stone tends to overwhelm the subtle interference colours. Pendants and crystal-point jewellery are the most common settings.

The iridescence is fragile in the sense that abrasion or recutting may breach the healed fracture and dull the effect. The hardness of quartz itself is unaffected at Mohs 7, but the optical phenomenon is surface-dependent within the stone and should be protected from heavy impact.

Distinction from related phenomena

Iris quartz is sometimes conflated with iris agate, but the two are mechanistically distinct: iris agate diffracts transmitted light through fine parallel banding, while iris quartz reflects light from internal thin films. Iris quartz also differs from aventurescent quartz, where the sparkle comes from oriented inclusions of mica or hematite, and from rutilated quartz, whose interest lies in the inclusions themselves rather than in any optical phenomenon.