Orthoclase Feldspar Gemstone & Information | Gemopedia by JTV | Gemopedia™
While feldspar is one of the most abundant mineral families in the world, gem quality crystals are scarce, coveted and spectacularly beautiful, as they often possess rare optical effects. Many feldspar gems only occur in isolated deposits and are far rarer than better known gems such as diamond, ruby or sapphire. Some varieties are commonly encountered and many are seen in granites, for example, "Black Pearl" granite countertops are actually composed mostly of feldspar. While orthoclase, a potassium-rich feldspar, is better known to collectors, there is one variety that many are familiar with--moonstone, prized for its shimmering adularescence.
Colorless, White, Green, Orange, Yellow To Brown, Gray
Orthoclase-feldspar classificationCommonNameOrthoclase FeldsparSpeciesOrthoclaseVarietyColorsColorless, White, Green, Orange, Yellow To Brown, GrayAlternateNamesAdularia, Moonstone, SunstoneGemstoneGroupsFeldsparKeySeparationsrefractive index, birefringence, optic character and possibly spectrum and fluorescenceClassificationCommentsFeldspar is a group of minerals of which orthoclase is one species. This information is illustrative of the yellow or orange-brown transparent faceted material depicted above, not necessarily other species and varieties within the group. Orthoclase feldspar can display a 4 rayed star.Adularia: colorless variety sometimes with adularescence.Moonstone: the semitransparent to opaque variety with white or blue adularescence. Sunstone: the semitransparent to opaque orange to brown variety that displays aventuresence.
Orthoclase-feldspar chemistry & crystallographyChemicalNamepotassium aluminum silicateChemicalFormulaKAlSi3O8SynthesisCrystalSystemMonoclinicChemistryClassificationSilicateNatureNaturalCrystallinityCrystallineChemistryComments
Orthoclase-feldspar optical propertiesTransparencyTransparent - OpaqueDispersionStrength: weak fire Value: 0.12OpticalCommentsRefractiveIndex1.518-1.526
Tolerance:(+0.010/-)0.010)Birefringence0.005-0.008OpticCharacterBiaxialOpticSignNegativePolariscopeReactionDoubly Refractive (DR)FluorescenceSWUV: transparent yellow: inert to weak reddish orange
LWUV: transparent yellow: inert to weak reddish orangesemitranslucent white: redCCFReactionPleochroismUnobservable
Orthoclase-feldspar characteristic physical propertiesHardness6-6.5CharacteristicCommentsStreakWhiteSpecificGravity2.55-2.61 Range:+/-0.03 Typical:2.58ToughnessPoorInclusionsOrthoclase feldspar might display centipede like inclusions. Material with aventuresence or asterism might contain reflective inclusions such as fine needles or elongated hematite platelets.LusterVitreousStabilityFractureUneven, SplinteryCleavagePerfect, in two directions