Isinglass, a rock-forming mineral (4)

I believe the answer is:

mica

(Other definitions for mica that I've seen before include "Isinglass" , "Shiny silicate mineral" , "Muscovite with perfect cleavage" , "Semitransparent insulating mineral" , "Flexible mineral found in igneous and metamorphic rocks" .)

I've seen this clue in The Telegraph.

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