Rainbow Moonstone Identification Guide

What Rainbow Moonstone Looks Like

"Rainbow moonstone" is, mineralogically, a near-colorless white labradorite (a plagioclase feldspar), not true orthoclase moonstone. The body is transparent to semi-translucent and milky-white, and the defining feature is a floating blue to multicolored sheen (adularescence/labradorescence) that drifts across the surface as you tilt it.

• Color: colorless to milky white body, with blue, violet, gold, and occasionally green flashes
• Luster: vitreous to pearly
• Transparency: transparent to translucent
• Habit: usually seen as polished cabochons, tumbles, or cleavage fragments; raw crystals are blocky

Step-by-Step Field ID Checklist

1. Tilt it under a single light source. Genuine rainbow moonstone shows a sheen that moves and changes color with the viewing angle, originating from inside the stone, not on the surface.
2. Look for internal cracks. Real material almost always shows fine internal feathery cleavage cracks ("centipede" inclusions) and a slight cloudiness.
3. Check the flash color. Blue and rainbow flashes appear only at certain angles, then vanish — unlike a printed or coated surface that glitters from everywhere.
4. Test hardness against quartz.
5. Look at cleavage on any broken edge.

Key Diagnostic Tests

• Mohs hardness: 6–6.5. It will scratch glass but a quartz point (7) will scratch it.
• Cleavage: two good cleavages meeting at nearly 90° — diagnostic of feldspar. Broken edges show flat, step-like planes, not the curved conchoidal fracture of glass or opal.
• Streak: white.
• Specific gravity: ~2.6–2.7, noticeably lighter than most colored gem garnets.
• Reaction to acid: none.

Common Look-Alikes and How to Tell Them Apart

• Opalite (man-made glass): the most common substitute. Opalite has a uniform milky blue glow from all angles and conchoidal fracture with no cleavage; under transmitted light it often looks orange/amber. Rainbow moonstone shows directional flash and feldspar cleavage.
• White precious opal: opal shows pinpoint play-of-color (discrete spectral flecks) rather than a single sweeping blue sheen, and is softer with no cleavage.
• White labradorite / true moonstone: these are genuinely close relatives; "rainbow moonstone" is simply transparent white labradorite. The trade distinction is the dominant blue rainbow sheen versus the softer billowy white-blue adularescence of orthoclase moonstone.
• Chalcedony or milky quartz: harder (7), no flash, no cleavage.

Where It Is Found

The premier source is southern India (Tamil Nadu/Karnataka), with additional material from Sri Lanka, Madagascar, and Tanzania. It forms in pegmatites and feldspar-rich igneous rocks.

Collector's Notes and Common Mistakes

The single most common error is buying opalite sold as "rainbow moonstone." Always do the tilt-and-flash test first: opalite's glow follows your light source uniformly, while moonstone's flash is locked to specific crystallographic angles and stays put as you change viewpoint. A second pitfall is confusing rainbow moonstone with labradorite — they are mineralogically the same plagioclase family, and the trade name simply describes light-colored, transparent material with strong blue rainbow flash. When grading, prize stones with a clean white body and a broad, vivid blue (or full-spectrum) sheen over those with a weak, patchy glow. Because cleavage is good in two directions, moonstone chips and cracks more easily than quartz; rough handling or ultrasonic cleaning can worsen those internal "centipede" feathers. For field collectors, watch feldspar-rich pegmatite dumps and weathered granite — blocky cleavage fragments that flash blue in sunlight are the giveaway.