diamanto

No participle here. Diamanto ends in -anto, which in Esperanto is the active present participle plus a noun ending: kanti gives kantanto a singer, labori gives laboranto a worker, helpi gives helpanto a helper. So the eye offers diami plus -ant-: one who diams. There is no verb diami. Diamant- is a single borrowed root and the -ant- in it is Greek grammar, fossilised, not Esperanto grammar working. This particular false seam catches people because the participle is genuinely everywhere - it is one of the first machines a learner is handed - and participles will not warn you which words were built with it.

The hook is that adamant and diamond are the same word. Greek adamas meant unconquerable, from a- (not) plus damao (I tame) - the same root that gives indomitable and, at a distance, Latin domare. The Greeks used it for the hardest substance they knew, and hardness became the name. Late Latin garbled adamantem into diamantem, the two spellings both survived, and English ended up with both: adamant, which kept the meaning unyielding, and diamond, which kept the stone. Esperanto took the stone as diamanto and the metaphor as nefleksebla or obstina, so it does not have the pun - but the pun is why an adamant person and a diamond ring are, historically, the same idea.

In use. Diamanta is the adjective, diamanteto a small stone, diamantisto a diamond cutter or dealer. Diamanta nupto is a diamond wedding. A cut stone is faceta - it has facetoj - and the sparkle itself is brilo, from brili, which is a real Esperanto build and a good one to set beside the false one at the top of this page.