A scarce polymorph of carbon is diamond (C). The physical properties of diamond and graphite couldn't be more different, considering they have the same chemistry (they're both just carbon). Diamond has a nonmetallic, adamantine luster, typically occurs in cubic or octahedral (double-pyramid) crystals, or subspherical to irregularly-shaped masses, and is extremely hard (H≡10). Diamonds can be almost any color, but are typically clearish, grayish, or yellowish. Many diamonds are noticeably fluorescent under black light (ultraviolet light), but the color and intensity of fluorescence varies. Some diamonds are phosphorescent - they glow for a short interval after a UV black light source is turned off.
Diamond (1.3 cm across) from Kasai-Oriental Province in central Zaire ("D.R. Congo"). This 7.23 carat, dark-brown diamond mass is a complex aggregate of several smaller diamond crystals. It comes from a late Late Cretaceous (65-79 m.y.) kimberlite pipe in the Mbuji-Mayi Kimberlite Field.
Diamonds from Africa - three yellowish-gray cubic diamonds & two clearish octahedral diamonds. Crystal at lower left is ~2.5 mm across.
Diamonds from Zaire (aka "D.R. Congo"). Each crystal is about 3 mm in size. These specimens don't significantly fluoresce under UV light.
Diamonds from Zaire (aka "D.R. Congo"). Crystals are ~1 to 1.5 mm in size.
Diamonds from Zaire (aka "D.R. Congo") (same lot as above) fluorescing under black light (UV). The green & magenta colors are from the diamonds. The bright blue specks are fibers (not geologic objects).
Diamonds from Zaire (aka "D.R. Congo") (same lot & same view as above) that are phosphorescent. No external light source is present. The only source of light here is from the diamonds, just after being subjected to UV light.
Diamonds from Russia. Crystals are about 0.7 to 0.9 mm in size.
Diamonds from Russia (same lot as above) fluorescing under black light (UV).
Diamonds from an undisclosed locality fluorescing under black light (UV) (above & below). Each crystal is on the order of 0.5 mm in size.
Diamonds phosphorescing after removal of a black light source. Same lot & same view as above.
Diamond from Australia - brown octahedral crystal from the 1.178 billion year-old Argyle Lamproite (Argyle Mine, nw Western Australia). The public has been tricked into buying ugly brown diamonds by their being marketed as "cognac diamonds". Light brown-colored diamonds are cunningly marketed as “champagne diamonds”.
Faceted diamond (5.5 mm across). The image on the right shows the famous "fire" of gem-quality diamond.
Left: illuminated by four light sources.
Right: illuminated by one light source.
Faceted diamond (~3.25 mm across).
Faceted diamonds in brooch (public display, Field Museum of Natural History, Chicago, Illinois, USA).