Chromium is steel-gray, lustrous, hard, metallic, and takes a high polish. Its compounds are toxic. It is found as chromite ore. Siberian red lead (crocoite, PrCrO4) is a chromium ore prized as a red pigment for oil paints.
Emerald is a form of beryl (a beryllium aluminium silicate) which is green because of the inclusion of a little chromium into the beryl crytal lattice in place of some of the aluminium ions. Similarly, traces of chromium incorporated into the crystal lattice of corundum (crystalline aluminium oxide, Al2O3) as a replacement for some of the Al3+ ions results in another highly coloured gem stone, in this case the red ruby.