Feldspar & the feldspathoids

What holds the family together

Feldspars are framework aluminosilicates with potassium, sodium or calcium in the structure, and they make up around sixty per cent of the earth's crust. The group splits into the potassium feldspars, including orthoclase and microcline, and the plagioclase series running from sodium-rich albite to calcium-rich anorthite.

The optical effects come from unmixing. As a feldspar cools slowly, its components can separate into alternating microscopic layers. Where those layers are around the wavelength of blue light, they scatter it, and you get the floating blue sheen of moonstone, called adularescence. Where the layers are thicker and reflect at specific angles, you get the peacock flashes of labradorite, called labradorescence. Where copper platelets are suspended in the crystal, you get the metallic glitter of sunstone, called aventurescence.

Two stones in this section are not feldspars in the strict sense. Sodalite and the lazurite in lapis lazuli are feldspathoids, formed where there was too little silica for true feldspar to crystallise. They are grouped here because that is where they belong in the crystal chemistry.

How it forms

Feldspar crystallises from cooling magma and is the dominant mineral in granite, in most volcanic rocks and in much metamorphic rock. Gem-quality material needs slow cooling and space, which usually means pegmatites.

Classic moonstone came from Sri Lanka and southern India, where it has been mined for centuries and where the finest blue material still originates. Labradorite was described from Labrador in 1770 and now comes mainly from Madagascar and Finland, whose particularly vivid variety is called spectrolite.

Amazonite is a green microcline coloured by trace lead, and its ancient sources include the Eastern Desert of Egypt and deposits in Ethiopia and Russia. Sunstone with copper inclusions is the state gem of Oregon and also comes from India.

Lapis lazuli forms in metamorphosed limestone, and the great source at Sar-e-Sang in Afghanistan has been worked for over six thousand years.

The stones of this family

• Moonstone (orthoclase & albite, Mohs 6). Adularescence, that blue sheen floating under the surface, comes from layers inside the crystal scattering light. Sacred to the moon in India, where it is a traveller's gift.
• Labradorite (plagioclase feldspar, Mohs 6). Grey until the light hits it, then peacock blue and gold. Named for Labrador in 1770, and central to Inuit accounts of the aurora trapped in stone.
• Amazonite (microcline with lead, Mohs 6). Blue-green microcline, worked in Egypt for the Book of the Dead and in Mesopotamia before that. Named for a river it has never been found beside.
• Sunstone (feldspar with copper platelets, Mohs 6). Aventurescent feldspar that flashes copper when turned. Oregon's state gem, and a Viking navigation candidate alongside Iceland spar.
• Lapis lazuli (lazurite with pyrite & calcite, Mohs 5.5). A rock, not a mineral, mined at Sar-e-Sang in Afghanistan for six thousand years and ground into ultramarine, once costlier than gold.
• Sodalite (Na₈Al₆Si₆O₂₄Cl₂, Mohs 5.5). Lapis without the gold flecks and at a fraction of the price. Named in 1811, so its lore is entirely modern, mostly about clear thinking.

Trade names and lookalikes

The main confusion is the word moonstone. Classic moonstone is orthoclase with albite layering and shows a blue or white float. Rainbow moonstone, which shows multicoloured flashes, is white labradorite, a plagioclase. The trade name is well established but the two are different minerals, and any account that claims a shared ancient tradition has conflated them.

Opalite is manufactured glass with a milky opalescence and is sold as moonstone, sea opal and a dozen other names. It is glass, and it is often not disclosed.

Amazonite is straightforward, though dyed green quartzite is sometimes substituted. Sunstone is imitated by goldstone, which is copper-bearing glass from seventeenth-century Italy.

Lapis lazuli is imitated by dyed jasper, sold as Swiss or German lapis, and by reconstituted powder in resin. Sodalite is often sold as lapis; the tell is that lapis has irregular gold pyrite flecks and sodalite has white veining and none.

Care that follows from the structure

This family is soft for its beauty. At 6 or below, feldspar scratches against quartz dust, which is present in ordinary household dust, so a polished piece dulls over years of casual handling.

Worse, feldspar cleaves in two directions at close to ninety degrees, which means a knock can split it cleanly rather than chip it. Moonstone in particular is notorious for this, and it is a poor choice for a ring worn while working.

Clean with a soft cloth and lukewarm soapy water. No ultrasonic cleaners, since the vibration can propagate a cleavage crack. Avoid sudden temperature changes.

Lapis needs special mention: it is porous, contains soft calcite, and is attacked by acids including sweat. Keep it away from cosmetics and never soak it.