Chromium

Chromium is a metallic chemical element (atomic number 24).  It is very hard and brittle, but as a result can take a very high polish, making it ideal for decorative uses.  
The body appears to use some traces chromium ions as part of the metabolism of sugars and fats.  Chromium deficiency is extremely rare and all reported cases were from patients who were only getting nutrition from an intravenous line.  However other ions are highly toxic and carcinogenic.  Toxic doses are measured in thousandths of a gram.  
Chromium is fairly common in the environment and has a number of practical uses.  A small amount of chromium in steel makes it very much harder, making it useful for cutting tools.  A larger amount is used to make stainless steel, which is common in everything from cutlery to scalpels.  Chromium is also used to coat softer and more corrosion succeptible metals.  
Chromium compounds are widely used as pigments.  The yellow color of school buses is due to chromium in the pigments.  Toxic compounds of chromium are replacing arsenic in treated wood products.  
Chromium at Wikipedia
Category:Chemistry