All gemstone are rare; some are just harder to find than others. In fact, there is no consensus on what is the rarest mineral or the rarest gemstone because there is no consensus on the definition of "rarity,"
according to the Gemological Institute of America. However, many of the stones in this series come from only one or two localities in the entire world, so in that sense, they are scarce. Opal is Australia’s national gemstone, and black opal is the rarest and most valuable of its kind, at times selling at prices that rival the best diamonds. The stone must have a rich, black background, but base colors come in all shades of gray, which is why opinions vary on what is a "true" black opal. Found in the Lightning Ridge area in northwestern New South Wales, black opals are natural, solid stones that absorb scattered white light, giving it brilliant spectral colors.
The Gemological Institure of America (GIA) calls musgravite "a rarity among the rare... a particular gem on our research examination 'want list.'" A very close relative of another hard-to-find gemstone, taaffeite (and often misidentified as such), musgravite was first discovered in 1967 in the Musgrave Range of South Australia. Facet grade -- the baseline measurement of how clean cut a sellable stone must be -- for musgravite was not reported until 1993. As of 2005, there were only eight musgravite specimens in the world.