Worried mates will hold your once semi-precious stone (8)
I believe the answer is:
amethyst
'semi-precious stone' is the definition.
'amethyst' can be an answer for 'stone' (I've seen this before). I'm unsure of the 'semi-precious' bit.
'worried mates will hold your' is the wordplay.
'worried' indicates an anagram (worry can mean to interfere with or disturb).
'will hold' means one lot of letters goes inside another (inserted letters are held inside).
'your' becomes 'thy' ('thy' historically meant 'your').
'mates' is an anagram of 'amest'.
'amest' enclosing 'thy' is 'AMETHYST'.
'once' acts as a link.
Can you help me to learn more?
(Other definitions for amethyst that I've seen before include "Purple gem" , "Something precious" , "Purplish gemstone" , "Purple quartz used in jewellery" , "Violet-coloured stone" .)