Pure gold belongs to the precious metals and is a dense but soft metal, slightly harder than zinc. For it can be used, it can be purified and, for most purposes, it must be alloyed with other materials to make it harder. The purity of gold jewelry is measured in carats; pure gold is 24 carats. Common levels of purity in the Netherlands are 14 carat (58.3% gold), 18k (75%) and 22 carat (91.7%). A mnemonic is one carat as 4% to approach pure gold. Gold has long been seen as a valuable metal. In Egyptian hieroglyphs from the 26th century BC. all mention of gold as payment. Until about the mid-20th century AD. Gold was still widely used for coins. In ancient times, gold was known not only as valuable, but there was magic and gave the symbol of purity. Alchemists have long been searching for the philosopher's stone, to transform other materials into gold. Because they never succeeded, it is easy to explain from the current understanding about the structure of atoms. In the 19th century there especially in the United States and Australia, many gold resources discovered, which are the gold rushes, Gold has been present on Earth since the creation of the planet itself, but it is not formed on Earth because for its production fusion reactions are needed. There are nuclear fusion reactions take place in the core of stars, but these fusion reactions do not provide enough energy for the production of gold. Stars that are heavy enough (from about 8 times the mass of the Sun) end their life in a supernova explosion. During this explosion there are formed by means of nuclear reactions heavy metals, including gold.