A mysterious shining substance, apparently invented by Fëanor himself, that he used to construct the Silmarils and trap the light of the Two Trees. The name silima derives from a root word meaning 'shine', especially with a white or silver light, and the word Silmaril comes in turn from the silima of which they were made. (The -ril element indicates 'brilliance', so in full the word Silmaril means 'brilliance of silima'.)
We know that the Silmarils were made of silima, and assuming that they were the only items constructed from the material (which seems likely, though cannot be established with certainty) then its secret was lost, at least to those in Middle-earth, after the end of the First Age.
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