FELDSPAR

FELDSPAR	Optical effects
Major Sources	Brazil, China, India, Madagascar, Sri Lanka, Tanzania, Tibet & USA
Colors Found	Various
Family	Feldspar
Hardness	6 to 6.5
Refractive Index	1.51–1.57; Biaxial (+ or –)
Specific Gravity	2.56–2.75
Crystal System	Monoclinic or triclinic
Enhancements	May be enhanced

While the feldspar is the most abundant mineral family in the world, gem quality crystals are scarce, coveted and spectacularly beautiful, often possessing rare optical effects such as adularescence, aventurescence and iridescence. Many feldspar gems only occur in isolated deposits and are far rarer than better known gems such as diamond, ruby or sapphire.

The name feldspar comes from the German feldt spat, meaning “field stone,” because when feldspar weathers, it releases plant nutrients, such as potassium, which enrich soil.

Just the facts

There are a wide variety of feldspar gem types and some are confusingly similar in appearance and composition. The various trade names can also confuse, as the same name is often applied to gemstones that are somewhat different in appearance, origin and composition. Amazonite, andesine, labradorite, moonstone, orthoclase, and sunstone are all members of the feldspar family.

The two main subgroups are the plagioclase group and the alkali (potassium) group.

ALKALI:	Composition	Notable Gem Examples
Orthoclase	Monoclinic: KAlSi3O8	Moonstone
Sanidine	Monoclinic: (K, Na)AlSi3O8	Rarely seen as a gem
Microcline	Triclinic: KAlSi3O8	Amazonite
Anorthoclase	Triclinic: (Na, K)AlSi3O8
PLAGIOCLASE:	Series from albite (NaAlSi3O8) through anorthite (CaAl2Si2O8) and includes:
Albite	100–90% albite	Moonstone
Oligoclase	90–70% albite	Sunstone, yellow, green
Andesine	50–70% albite	Sunstone
Labradorite	30–50% albite	Spectrolite, moonstone, sunstone, colorless, yellow, red, green
Bytownite	10–30% albite	Reddish, rarely cut
Anorthite	0–10% albite	Pale yellow, rarely cut