Excited mate’s holding your old gemstone (8)
I believe the answer is:
amethyst
'gemstone' is the definition.
(amethyst is a kind of gemstone)
'excited mate's holding your old' is the wordplay.
'excited' is an anagram indicator.
'holding' indicates putting letters inside.
'your old' becomes 'thy' ('thy' historically meant 'your').
'mates' anagrammed gives 'amest'.
'amest' going around 'thy' is 'AMETHYST'.
(Other definitions for amethyst that I've seen before include "Purple quartz used in jewellery" , "Violet-coloured stone" , "Semiprecious stone of violet or purple quartz" , "A Middle East language" , "One showing sparkling facets" .)