Card showing precious stone (7)
I believe the answer is:
diamond
I believe this clue is a double definition.
'card' is the first definition.
(I know that diamond is a type of playing card)
'precious stone' is the second definition.
(I know that diamond is a type of precious stone)
'showing' acts as a link.
(Other definitions for diamond that I've seen before include "shape" , "rock hard" , "Very hard substance" , "One of her best friends" , "Gemstone - baseball field" .)