The Color Black
Always Black: Black Tourmaline (Schorl), Jet, Obsidian.
Sometimes Black: Agate, Diamond, Jasper, Onyx, Spinel
Black Star Diopside and Black Star Sapphire are not really black but so extremely dark that they are named so. Extremely dark Garnets and Amethysts also appear black sometimes with rich and subtle color tones, while “black” Pearls, Opals and Diamonds stretch the meaning of what a black gem is.
Though often considered black, natural Hematite is a dark metallic grey or silvery black, as is Marcasite. Onyx is often assumed to be a solid black gemstone because it's commonly dyed. In nature it's actually a kind of banded agate. Tourmalinated Quartz, while mostly a transparent “white” gem, is unique because of its needle-like inclusions of Black Tourmaline.
Though not a gemstone, Black Lava is sometimes used in jewelry, as are black natural components like shell and wood, and the various cords such as hemp.