Chromium's atomic number is 24 and its symbol is Cr. Chromium metal is well known for its high silvery luster that is an icon of luxury. Chromium is also the world's hardest metal. Because chromium is very hard and resistant to corrosion it is a key additive in steel. On its own chromium is very brittle, which is why it's often alloyed with other metals or only used as a thin outer layer for looks. Chromium also plays an important role in human biology for insulin, sugar, and lipid metabolism. Chromium is capable of creating compounds that create vivid colors such as red, yellow, and purple. In fact, chromium is what is responsible for giving emeralds their green color and rubies their red color. The vibrancy of red in rubies is dependent on how much chromium is present in the gemstone.
