Luster

Classifying Light Reflection

Luster describes how light interacts with the surface of a crystal, rock, or mineral. It is a function of the refractive index and the texture of the surface. Identifying the correct luster is often the first step in mineral identification, separating minerals into two broad categories: <strong>Metallic</strong> and <strong>Non-Metallic</strong>.

Common Types of Luster

• Metallic: Looks like polished metal (e.g., Pyrite, Galena, Gold). It is opaque and reflective.
• Sub-metallic: Looks like dull or tarnished metal (e.g., Hematite, Magnetite).
• Vitreous: The most common luster, looking like broken glass (e.g., Quartz, Topaz, Tourmaline).
• Adamantine: An incredibly brilliant, diamond-like shine caused by a high refractive index (e.g., Diamond, Cerussite).
• Resinous: Looks like resin or plastic (e.g., Amber, Sphalerite).
• Pearly: Iridescent like the inside of a seashell (e.g., Moonstone, Talc).
• Silky: A fibrous shine (e.g., Gypsum, Asbestos).