From Fallen London Wiki
Why are diamonds so hard to find down here? Does Mr Stones really keep them for itself?
You can acquire these in various opportunities around the city. If you are A Person of Some Importance - you can also buy them at Empire Adornments in the Bazaar Sidestreets.
See here for a list of how to obtain this item, or click to show them.
It can be sold at the Bazaar:
It can be purchased from Merry Gun's Exchange in The Rat Market, but cannot be sold there.
See here for a list of pages which require this item, or click to show them.
See Contraband Grinding (Guide) for information on grinding this item.