Anorthoclase: Gemstone Information 

K-analbite, K-monoalbite, K-albite (also see larvikite). Anorthoclase is the name given to a member of the alkali feldspar series that contains more than two-thirds sodium and has triclinic symmetry. Anorthoclase sometimes shows twinning, but generally not the multiple twinning seen in plagioclase.

Occurrence
As phenocrysts in high-temperature volcanic rocks such as trachytes, phonolites and andesites and in tuffs. Rather abundant worldwide. Some localities for well-characterized material include: Pantelleria and Ustica Islands, Italy; Larvik, Norway; Berkum, North Rhine-Westphalia, Germany; Kilju, Hamgyongbukto, Democratic People’s Republic of Korea; Mt. Erebus, Ross Island, Antartica; Camargo, Chihuahua, Mexico; Stettin, Marathon Co., Wisconsin, USA; Somali Republic; Australia and New Zealand.
Appearance
Opaque to translucent , rarely transparent
• Lustre: Vitreous, may be pearly on cleavages
• Colour: Colorless, also white, pale creamy yellow, occasionally greenish or pink, red.
Properties
• Crystal system: Monoclinic
• Refractive index: α 1.518–1.527; β 1.522–1.532; γ 1.522–1.534 Biaxial ve
• Birefringence: 0.006–0.008
• Pleochroism: Weak
• Density: 2.56–2.62
• Hardness: 6–6.5
• Dispersion: Weak
• Cleavage/fracture: Perfect to good, prismatic partings. Uneven to conchoidal fracture
• Optical effects: chatoyancy, asterism, colour change, ADR See moonstone – Australia.