Mohs Hardness 6 to 6.5: Feldspar, Moonstone and Labradorite

Feldspar is the most abundant mineral group in Earth's crust, and several of its varieties are prized as gemstones. Moonstone, labradorite, sunstone, and amazonite are all feldspar gems with a hardness of Mohs 6 to 6.5. This places them below the critical Mohs 7 dust threshold, meaning they require more careful handling and protective settings than harder gems. Their extraordinary optical phenomena, however, make them among the most visually captivating gems available.

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The Feldspar Group

Feldspar minerals are aluminum silicates with potassium, sodium, or calcium. The two main feldspar subgroups relevant to gemology are orthoclase (potassium feldspar, which includes moonstone and adularia) and plagioclase (sodium-calcium feldspar, which includes labradorite, sunstone, and peristerite). All gem feldspars share a hardness of Mohs 6 to 6.5 and two directions of perfect cleavage at nearly right angles to each other.

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Moonstone

Moonstone is orthoclase feldspar that displays adularescence, a floating blue or white glow that moves across the stone as it is rotated. This optical phenomenon is caused by light scattering between alternating layers of two different feldspar minerals (orthoclase and albite) within the stone. The finest moonstone shows a vivid blue adularescence against a colorless or near-colorless body.

Moonstone has Mohs 6 hardness and two directions of perfect cleavage, making it one of the more fragile gem materials in common use. It scratches from everyday dust and contact and can split from impact along its cleavage planes. Moonstone is best suited for earrings and pendants rather than rings. If used in rings, bezel settings provide essential protection. Clean only with a soft damp cloth. Avoid ultrasonic cleaners, which can cause cleavage fractures.

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Labradorite

Labradorite is plagioclase feldspar that displays labradorescence, a spectacular iridescent play of color in blue, green, gold, orange, and red caused by light interference between thin layers within the crystal. Spectrolite is a trade name for exceptionally vivid labradorite from Finland. Rainbow moonstone is actually white labradorite with blue adularescence.

Labradorite has Mohs 6 to 6.5 hardness and the same two directions of perfect cleavage as moonstone. Care requirements are identical: protective settings, gentle cleaning with a soft damp cloth, no ultrasonic cleaners, and careful handling to avoid impact.

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Sunstone

Sunstone is feldspar (usually oligoclase or orthoclase) that contains tiny platelets of hematite, goethite, or copper that reflect light and create a glittering effect called aventurescence. Oregon sunstone is a particularly prized variety that contains copper platelets and occurs in red, orange, yellow, green, and bicolor combinations. Sunstone has the same Mohs 6 to 6.5 hardness and cleavage as other feldspars.

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Amazonite

Amazonite is green to blue-green microcline feldspar colored by lead and water. It has been used as a gem and ornamental stone since ancient Egypt. Amazonite has Mohs 6 to 6.5 hardness and the same cleavage as other feldspars. It is typically used in cabochons and beads rather than faceted stones.

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Caring for Feldspar Gems

All feldspar gems share the same care requirements due to their similar hardness and cleavage. Clean with a soft damp cloth only. Avoid ultrasonic cleaners, steam cleaners, and soaking. Use protective bezel settings for rings. Remove feldspar jewelry before any physical activity. Store in individual soft pouches away from harder gems. Handle carefully to avoid impact, which can cause cleavage fractures even in well-set stones.

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Frequently Asked Questions

Can I wear moonstone in a ring every day?

It is not recommended. Moonstone's Mohs 6 hardness means it will accumulate surface scratches from everyday dust, and its perfect cleavage makes it vulnerable to chipping from impact. If you want to wear moonstone in a ring, choose a bezel setting, accept that the surface will show wear over time, and have it re-polished periodically.

What causes the blue glow in moonstone?

The blue adularescence in moonstone is caused by light scattering between alternating layers of orthoclase and albite feldspar within the stone. The thickness of these layers determines the color of the adularescence: thinner layers produce blue, thicker layers produce white or silver.

Is labradorite the same as moonstone?

No. Both are feldspar gems with similar hardness and optical phenomena, but they are different minerals. Moonstone is orthoclase feldspar showing adularescence. Labradorite is plagioclase feldspar showing labradorescence. Rainbow moonstone is actually white labradorite, which causes some confusion.

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Conclusion

Feldspar gems including moonstone, labradorite, sunstone, and amazonite are among the most visually spectacular gem materials available, with optical phenomena that no other gem family can match. Their Mohs 6 to 6.5 hardness and perfect cleavage require careful handling and protective settings, but with appropriate care these gems reward their owners with extraordinary beauty.