Expensive gems and card suit (8)
I believe the answer is:
diamonds
'expensive gems and card suit' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for diamonds that I've seen before include "Precious gems" , "where some Americans find sporting action" , "Pieces of crystalline carbon valued as gems" , "Gemstones and card suit" , "Musically, they're a girl's best friend" .)