(IDEX Online) - An extremely rare 93-ct black diamond has gone on display at London's Natural History Museum.The rough diamond, which weighed over 300 carats, is thought to have originated in Brazil. It was acquired by the lender's family during the late 1800s in Goa, India - a well-known gem cutting center at the time.The stone was cut and polished, and set into a white gold pendant with an Arabic-inspired design, including 41 colorless and 26 yellow diamonds on the rear, depicting the as Ursa Major constellation.Robin Hansen, gems curator at the museum, said: "This gemstone appears black because it is made up of tiny individual diamond grains that are intergrown, known as polycrystalline. It also contains many minute inclusions of other minerals, mostly black graphite."The boundaries between the different grains, as well as these inclusions prevent light from passing through the diamond, so although it is partly translucent, it appears opaque black. The inclusions also create a glitter-like effect inside the diamond."Pic courtesy Natural History Museum