Like sapphires, rubies are formed from corundum. Rubies are coloured red due to chromium, and they are often valued based on the intensity of their red colour. The fact that some sapphires can appear red due to impurities in the crystal, and that there are other red gemstones that exist such as spinel, it is easy to mistake less valuable red gems for rubies. Rubies are usually cut into an oval shape, unlike other gemstones which are usually angular shapes. In the United States, some pink rubies are called pink sapphires, as there are different scales for determining the purity and value of rubies.