page_title: Lustre (mineralogy)
section_title: Asterism
hierarchical_section_title: Lustre (mineralogy) / Optical phenomena / Asterism
caption_attribution_description: English: Star sapphire cabochon displaying six-ray asterism. Français&#160;: Saphir étoilé, taillé en cabochon pour mettre en evidence son astérisme.
context_page_description: Lustre or luster is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin lux, meaning "light", and generally implies radiance, gloss, or brilliance.
  A range of terms are used to describe lustre, such as earthy, metallic, greasy, and silky. Similarly, the term vitreous refers to a glassy lustre. A list of these terms is given below.
  Lustre varies over a wide continuum, and so there are no rigid boundaries between the different types of lustre. The terms are frequently combined to describe intermediate types of lustre.
  Some minerals exhibit unusual optical phenomena, such as asterism or chatoyancy. A list of such phenomena is given below.
context_section_description: Asterism is the display of a star-shaped luminous area.  It is seen in some sapphires and rubies, where it is caused by impurities of rutile.  It can also occur in garnet, diopside and spinel.
caption: Sapphire cabochon
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