Rainbow iridescence originally seen in various types of mineral (7)
I believe the answer is:
pyrites
'mineral' is the definition.
(I have seen 'Shiny mineral ' mean 'pyrites' so perhaps 'mineral' could also mean 'pyrites')
'rainbow iridescence originally seen in various types' is the wordplay.
'originally' indicates taking the first letters.
'seen in' is an insertion indicator.
'various' indicates an anagram.
The first letters of 'rainbow iridescence' is 'ri'.
'types' is an anagram of 'pytes'.
'ri' going within 'pytes' is 'PYRITES'.
'of' is the link.
(Other definitions for pyrites that I've seen before include "Shiny yellow mineral" , "Iron-rich mineral" , "Mineral such as fool's gold" .)