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|Is a Variety of||Feldspar|
These feldspars are rarely encountered in gem form. Their occurrence is widespread throughout the world, in a great variety of rock types and environments, but in most cases transparent crystals are rare.
In many cases faceted gems are identified as a feldspar in the plagioclase series, but the finder does not have the instrumentation needed to pin down the species. This is accomplished by a combination of optical and X-ray analysis. A few plagioclase gems have been well characterized, however, and reported in the literature.
Oligoclase comes from Greek words meaning little break because the cleavage was believed to be less perfect than in albite.
Oligoclase is reported from the Hawk Mine, Bakersville, North Carolina, in colorless to pale green facetable crystals. The indices are 1.537-1.547; density 2.651. Oligoclase is also reported from Kenya, colorless grains, with indices as follows: a = 1.538-1.540,β= 1.542-1.544, γ= 1.549- 1.550; birefringence 0.010-0.011; S.G. 2.64. North Carolina gems up to about 1-5 carats, colorless to pale green, are reported. Oligoclase from Baffin Island, N.W.T., Canada has yielded cut gems up to about 5 carats.