Ruin ostentatious jewellery's mottled appearance (8)
I believe the answer is:
marbling
'mottled appearance' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'ruin ostentatious jewellery's' is the wordplay.
'ruin' becomes 'mar' (I've seen this before).
'ostentatious jewellery' becomes 'bling' (I've seen this before).
'mar'+'bling'='MARBLING'
Can you help me to learn more?
(Other definitions for marbling that I've seen before include "Mottled effect" , "Streaky lines" , "Distribution of fat" .)