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.
Aluminum silicate of potassium, sodium and calcium. KAlSi
. Isomorphous series made up of Albite (Na), Oligoclase, Andesine, Labradorite, Bytownite and Anorthite (Ca) where the two end member are NaAlSi
Colors / Varieties:
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:
2.63 - 2.72
Cleavage / Fracture:
Easy 2 directional / Uneven.
; Biaxial negative
Refractive Index / Birefringence:
1.542 - 1.549 / 0.00
Platelets of goethite, copper,
, fingerprints and crystal inclusions.
Cause of Color:
Aventurescent effect due to reflections from metallic inclusion like
, goethite or copper.
Simulants (with separation tests):
Sunstone simulated by aventurine
(structure), goldstone glass (inclusions).
Igneous rocks, pegmatite.
(Oregon), India, Kenya, Norway, Russia.
Cuts & Uses:
Facetted, cabochon, beads and carving.