Precious stones found in suit (8)
I believe the answer is:
diamonds
I believe this clue is a double definition.
'precious stones' is the first definition.
(diamond is a kind of precious stone)
'suit' is the second definition.
(I've seen this before)
'found in' is the link.
(Other definitions for diamonds that I've seen before include "Expensive gems and card suit" , "Quarter-deck" , "Maids don these stones" , "In song they're a girl's best friend" , "Red cards" .)