Diamond is the hardest known material, which is formed deep in the Earth's crust in a process that lasts for several million years. The youngest diamond in the world is app. 5 million years old. Why is its price so high? Diamond is not only expensive, because it is rare and has a beautiful shine, but also because it forms deep underground and the excavation of diamonds requires a number of machines and people (as an interesting fact, by excavating 1t of mine, we get a diamond the size of 1ct). Out of all excavated diamonds, a very small percent is such that is suitable for future grinding and use in the jewellery business. Zlatarna Celje issues a certificate for each piece of jewellery with engraved stones, which contains information on the diamond and confirms its quality.