Carlomaria Grassi ||| BBC - Earth - How the world's biggest sapphire grew so large

… Making sapphires is a lengthy process. This
particular sapphire would have formed within the rocks of Sri Lanka's
highlands. The temperatures and
pressures would have changed only very slowly over millions and millions of
years The rocks are mostly granites, which form when molten
magma cools and solidifies. But they have been subjected to intense heat and
pressure. The granites have been dated as almost two billion years old, and
were subsequently squeezed and re-worked in a massive metamorphic
mountain-building episode that happened more than 500 million years ago. Temperatures
and pressures deep within the roots of these mountains would have reached more
than 900C and over 9000 atmospheres pressure  ...   BBC - Earth - How the world's biggest sapphire grew so large