Stone in part of baseball playing area (7)
I believe the answer is:
diamond
I believe this is a double definition.
'stone' is the first definition.
(I know that diamond is a type of transparent gem)
'part of baseball playing area' is the second definition.
(area on a baseball field)
'in' is the link.
(Other definitions for diamond that I've seen before include "Form of carbon" , "Playing card - baseball playing-field" , "Precious stone of pure crystallized carbon" , "American pitch" , "suited one" .)