I converted this lovely labradorite monkey head (which was on a bar brooch) using an antique (early 19th century) ring shank that I had stored away.
The band is a simple wide band with split shoulders that are engraved with the classic acanthus pattern.
 
size - 4.5 , but can be sized
weighs - 3.9g
Monkey head english circa 1870
back of the band measures - 4mm 
monkey head is about 12mm x 13mm
band tests as 9 carat
 
in excellent condition