Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium. In many cultures of the Old and New Worlds, this gemstone has been esteemed for thousands of years as a holy stone, a bringer of good fortune or a talisman. The oldest evidence for this claim was found in ancient Egypt, where grave furnishings with turquoise inlay were discovered, dating from approximately 3000 BC. In the ancient Persian Empire, the sky-blue gemstones were earlier worn round the neck or wrist as protection against unnatural death. If they changed color, the wearer was thought to have reason to fear the approach of doom. Meanwhile, it has been discovered that turquoise can change color. The change can be caused by light, or by a chemical reaction brought about by cosmetics, dust or the acidity of the skin.