Musgravite is so rare that examples of them selling are difficult to come by. There's one for sale at Gem Rock Auctions which is billing itself as the largest musgravite in the world at 16 carats.

It is one of the rarest gemstones on Earth ... an auction where he sold two Tanzanite rocks for $3.4m After his June sale of two rocks weighing 9.2kg and 5.8kg, Mr Laizer - a father of more ...