Gemstone profile and identification guide
Orthoclase
Orthoclase is a monoclinic potassium feldspar species and an important member of the moonstone feldspar series.
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Overview
Orthoclase is a monoclinic potassium feldspar species and an important member of the moonstone feldspar series.
Did you know? Orthoclase is one of the major feldspar minerals that dominate Earths continental crust.
Use this profile as a collector reference and educational guide for quick on-page comparison, then validate with the interactive identification system.
Identification
Name: Orthoclase
Scientific classification: Silicate (Feldspar: Orthoclase)
Hardness: 6 Mohs
Transparency: transparent
Lustre: vitreous
Crystal system: Monoclinic
Primary colour: Colorless
Secondary colour: Yellow
Colour family: Whites & Neutrals
Feldspar RI/SG values, monoclinic symmetry, and cleavage behavior identify orthoclase.
Verification Status
Status: verified-core-fields
Reference priority: GIA, Gem-A, Mindat, International Gem Society
Core identity, locality, and physical-property fields reviewed against standard gemological references.
Values are reported as practical gemological ranges where appropriate.
Physical Properties
Hardness category: hard
Hardness value: 6 Mohs
Chemical composition: Potassium aluminum silicate (KAlSi3O8)
Crystal system: Monoclinic
Common locations: Madagascar, Sri Lanka, India, Brazil, Myanmar
Learn more: Gem-A gemstone education resources.
Colour Variations
Colorless, white, yellow, or pink potassium feldspar in transparent to translucent forms.
Optical effects: None specified in the current dataset.
Detailed Gemology
Mineral species: Orthoclase
Mineral group: Feldspar group
Chemical formula: Potassium aluminum silicate (KAlSi3O8)
Mohs hardness: 6
Crystal system: Monoclinic
Lustre: vitreous
Transparency: transparent
Streak: White
Cleavage: Perfect in two directions
Fracture: Uneven
Specific gravity: 2.55-2.63
Refractive index: 1.518-1.526
Optical phenomena: None typically diagnostic
Colour range: Colorless, white, yellow, pink
Common inclusions: Twinning, perthitic textures, fractures
Geological environment: Granitic and pegmatitic igneous environments
Principal mining regions: Madagascar, Sri Lanka, India, Brazil, Myanmar
Important commercial sources: Gem orthoclase and orthoclase-derived moonstone material are sourced from Sri Lanka, Madagascar, and other feldspar localities
Historical significance: Fundamental mineral in petrology and feldspar gem trade.
Common treatments: Generally untreated.
Synthetic or imitation forms: Glass imitations may be sold for low-cost products.
Common look-alikes: Sanidine, moonstone, quartz
Jewellery uses: Used in faceted gems, collector specimens, and feldspar study collections.
Care recommendations: Orthoclase should be cleaned with lukewarm water, mild soap, and a soft brush. At Mohs 6 in the Monoclinic system, avoid steam cleaners, harsh chemicals, and high-impact wear. Protect cleavage directions from sharp knocks during wear and setting.
Cross-check profile data with GIA Gem Encyclopedia and Mindat entry for Orthoclase.
Formation
Crystallizes in granitic and pegmatitic igneous rocks and associated hydrothermal systems.
Origin
Gem orthoclase and orthoclase-derived moonstone material are sourced from Sri Lanka, Madagascar, and other feldspar localities
Uses
Best for: Used in faceted gems, collector specimens, and feldspar study collections.
Ideal uses: Collector faceting and low-impact jewelry use.
Care
Orthoclase should be cleaned with lukewarm water, mild soap, and a soft brush. At Mohs 6 in the Monoclinic system, avoid steam cleaners, harsh chemicals, and high-impact wear. Protect cleavage directions from sharp knocks during wear and setting.
Need another candidate? Use advanced gemstone search or return to the identification guide.
Related gemstones by colour
Related gemstones by hardness
Commonly confused gemstones
Orthoclase FAQ
How can I identify Orthoclase?
Compare hardness (6 Mohs), crystal system (Monoclinic), colour profile, and lustre (vitreous) with your sample. Then cross-check related gemstones listed above.
What colour range does Orthoclase appear in?
Colorless, white, yellow, or pink potassium feldspar in transparent to translucent forms.
Where is Orthoclase commonly used?
Collector faceting and low-impact jewelry use.. For maintenance, follow the care guidance in this profile.
Scientific References
GIA: general gemstone identity (reviewed) - Batch 04 factual verification pass.
Gem-A: gemological terminology and testing references (reviewed) - Batch 04 factual verification pass.
Mindat: mineral species and principal localities (reviewed) - Batch 04 factual verification pass.
International Gem Society: gem trade context and treatment overview (reviewed) - Batch 04 factual verification pass.