Sunstone

Oligoclase is a species of plagioclase feldspar. The variety used in jewellery is called sunstone or, less commonly, aventurine feldspar. It has reflective inclusions of red, orange, or green platy crystals, which give it a metallic glitter. Sunstone might be faceted or carved - often as cabochons.

Sunstone occurs in metamorphic and igneous rocks in Norway, United States, India, the former USSR, and Canada.



Sunstone Properties

Chemical Composition: 
Aluminum silicate of potassium, sodium and calcium. KAlSi3O8. Isomorphous series made up of Albite (Na), Oligoclase, Andesine, Labradorite, Bytownite and Anorthite (Ca) where the two end member are NaAlSi3O8 and CaAl2Si2O8.
Colors / Varieties: 
Plagioclase:
  • Albite: Moonstone (orthoclase-albite)
  • Labradorite: Transparent (white, grey, yellow); Phenomena: chatoyancy, asterism, play of color i.e., labradorescence); Spectrolite: dark background with play of color.
  • Oligoclase: Sunstone: Phenomena: aventurescent, chatoyancy
Crystal System / Forms: 
Triclinic System
Hardness: 
6.5
Specific Gravity: 
2.63 - 2.72
Cleavage / Fracture: 
Easy 2 directional / Uneven.
Optic Character: 
Anisotropic, D.R.; Biaxial negative
Lustre: 
Vitreous
Refractive Index / Birefringence: 
1.542 - 1.549 / 0.00
Pleochroism: 
Not common.
Dispersion: 
0.012
Magnification: 
Platelets of goethite, copper, hematite, fingerprints and crystal inclusions.
U.V. Fluorescence: 
Variable.
Spectrum: 
Not characteristic.
Cause of Color: 
Aventurescent effect due to reflections from metallic inclusion like hematite, goethite or copper.
Treatment (Enhancement): 
None.
Synthesis: 
None.
Simulants (with separation tests): 
Sunstone simulated by aventurine quartz (structure), goldstone glass (inclusions).
Geological Occurrence: 
Igneous rocks, pegmatite.
Sources: 
U.S.A. (Oregon), India, Kenya, Norway, Russia.
Cuts & Uses: 
Facetted, cabochon, beads and carving.