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Even without considering other planets, diamonds are not all that rare compared to other gemstones. The De Beers company used a number of methods, some rather shady and unsavory, to artificially inflate their value.
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Very true. 
There are four "green sand" beaches in the world.  The green comes from olivine (peridot) mixed in with the sand.  Tiny crystals, to be sure, but plentiful.