Peridot is a gem-quality form of the mineral olivine. Peridot is known as an ‘idiochromatic’ stone, which means that its colour is not due to chemical impurities, as with stones such as sapphire and ruby (known as ‘allochromatic’), but is instead intrinsic to the stone, and caused by its fundamental chemical composition. In the case of peridot, that colour is a distinctive, rich yellowish green caused by iron, with minor variations in tone depending on the percentage of iron in the crystal lattice. Historically, peridot was famously sourced from the isle of St John in the Red Sea, but in the modern era, its principal source is Arizona in the US, but there are also deposits in Australia, Brazil, Myanmar and elsewhere. Most surprisingly, gem-quality peridot has been found in rare pallasite meteorites that have fallen to earth. Peridot is a vivid and distinctive stone, with high lustre and reasonable hardness, and is also the birthstone for the month of August.