Silly mates hiding your old gemstone (8)
I believe the answer is:
amethyst
'gemstone' is the definition.
(amethyst is a kind of gemstone)
'silly mates hiding your old' is the wordplay.
'silly' indicates anagramming the letters.
'hiding' is an insertion indicator.
'your old' becomes 'thy' ('thy' historically meant 'your').
'mates' is an anagram of 'amest'.
'amest' enclosing 'thy' is 'AMETHYST'.
(Other definitions for amethyst that I've seen before include "February stone" , "Precious stone of purple quartz" , "Variety of quartz" , "Semiprecious stone of violet or purple quartz" , "Purple quartz used in jewellery" .)