Gems from royal rings are on display outside (6)
I believe the answer is:
pearls
'gems from royal' is the definition.
'pearls' can be an answer for 'gems' (pearl is a kind of gem). I'm not certain of the 'from royal' bit.
'rings are on display outside' is the wordplay.
'rings' becomes 'peals' ('peal' can be a synonym of 'ring').
'are on' indicates putting letters inside (as in clothing 'on' a person).
'display outside' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'peals' enclosing 'r' is 'PEARLS'.
Can you help me to learn more?
(Other definitions for pearls that I've seen before include "Drops" , "Don't cast these jewels before swine" , "Oyster jewels" , "Jewels from bi-valves" , "Oyster gems" .)