Crooked team nicking your old small gemstone (8)
I believe the answer is:
amethyst
'gemstone' is the definition.
(amethyst is a kind of gemstone)
'crooked team nicking your old small' is the wordplay.
'crooked' is an anagram indicator.
'nicking' means one lot of letters goes inside another (I've seen this in other clues).
'your old' becomes 'thy'.
'small' becomes 's' (abbreviation - e.g. clothes size).
'team' with letters rearranged gives 'amet'.
'thy'+'s'='thys'
'amet' going around 'thys' is 'AMETHYST'.
(Other definitions for amethyst that I've seen before include "Type of quartz" , "One showing sparkling facets" , "Violet gemstone" , "February stone" , "Precious stone of purple quartz" .)