Big chunks of biotite are dark brown and seem opaque. A few minerals, such as goethite and sphalerite, show submetallic luster. We may not be able to detect this on thin edges with the eyes, but the light penetration is enough so that the mineral, though dark, does not have the typical surface appearance of a metal. Non-metallic minerals are commonly subdivided into different luster categories due to the way light behaves when it hits the mineral's surface, or as it passes through the mineral. The most common non-metallic lusters are adamantine, vitreous, resinous, greasy, and pearly.
Adamantine luster is the brilliant luster for diamonds and other gemstones.
The sparkle originates from the slow speed of light through the mineral and the way it is bent as it passes through. Diamond cuts are angled to take advantage of this and focus and emphasize the light rays still more. Vitreous, or glassy, lusters do not bend or slow the light as much, producing a shiny, but not spectacular, gleam. A resinous luster has the appearance of broken shellac and is due to a further slowing of the light. Greasy lusters are due to the light interacting with a surface film on the mineral.