Oligoclase: Gemstone Information 

As a major part of many rocks, especially igneous rocks such as granites, syenites and pegmatites. Also in intermediate lavas, schists and some amphibolites. Some major localities are: Sri Lanka; USA – Oregon sunstone
(localities in New York, Maine, New Mexico, North Carolina – pale faceted stones from Hawk mica mine, Bakersville {circa 1887} RI 1.537–1.547; SG 2.651, Pennsylvania and Virginia are relatively unimportant); Brazil;
Kenya (colourless with a blue or yellow tinge – RI range α 1.535–1.540, β 1.539–1.544, γ 1.544–1.550, DR 0.010–0.011; SG 2.63–2.64, Russia (plagioclases of Dlinnolambinskoye pegmatitic field, Kareliya); Sweden, Norway
and Canada (Baffin Island gems up to 5 cts). Gems, other than moonstone and sunstone (sensu stricto), are uncommon but can be large.
• Lustre: Vitreous to dull
• Colour: Is usually off-white or grey or pale shades of green, blue, yellow or brown
• Varieties: Sunstone is the name given to oligoclase with reflective hematite (or goethite) inclusions (aventurine-feldspar), peristerite is the name given to iridescent albite-oligoclase lamellar intergrowths
(see also peristerescence and moonstone), the term ‘moonstone’ has been applied to both peristerite and alkali feldspars with perthitic intergrowths that exhibit sheen
• Other: Lamellar twinning is ubiquitous and may cause a grooved effect on crystal and cleavage surfaces that appear as striations.
• Crystal system: Triclinic
• Refractive index: α 1.533–1.543; β 1.537–1.559; γ 1.542–1.552 Biaxial ve
• Birefringence: 0.009
• Density: 2.64–2.66
• Hardness: 6–6.5
• Dispersion: Weak
• Cleavage/fracture: Perfect in one and good in another direction forming nearly right-angled prisms/conchoidal fracture
• Absorption spectra
• Fluorescence: LWUV – pale brownish white, pale brown. Kenyan material fluoresces a faint white under LWUV and is inert under SWUV
• Inclusions: Hematite, goethite.
• Optical effects: chatoyancy, asterism, colour change, ADR etc. Sunstone effect
Treatment: Irradiation may turn colourless material blue if lead and water impurities are present.
Oligoclase has also been reported to turn orange and green upon irradiation.