Rebuilt Gretna’s a gem (6)
I believe the answer is:
garnet
'a gem' is the definition.
(I know that garnet is a type of transparent gem)
'rebuilt gretna's' is the wordplay.
'rebuilt' indicates an anagram.
'gretna' with letters rearranged gives 'GARNET'.
(Other definitions for garnet that I've seen before include "Deep red gemstone" , "Deep red precious stone" , "Deep red gem stone" , "Semi-precious stone of deep transparent red" , "Glassy red, yellow or green gemstone" .)