Oligoclase Value, Price, and Jewelry Information

FELDSPAR: Oglioclase, Canada (3.96, 7.21). Photo © Joel E. Arem, PhD, FGA. Used with permission.

Oligoclase

Oligoclase Information

Data Value
Name Oligoclase
Is a Variety of Feldspar
Colors Colorless, white, pale yellow, orange, pale green, blueish, reddish, brown, gray
Fracture Conchoidal, uneven, splintery
Hardness 6-6.5
Cleavage Perfect 2 directions
Crystallography Triclinic.
Refractive Index 1.537-1.550
Birefringence 0.007-0.011
Absorption Spectrum Not diagnostic.
Pleochroism None.
Optics Biaxial (-). 2V = 82°.
Optic Sign Biaxial -
Luster Vitreous.
Specific Gravity 2.62-2.67
Transparency Transparent.

Comments

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.

Name

Oligoclase comes from Greek words meaning little break because the cleavage was believed to be less perfect than in albite.

Occurrence

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.

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