It was listed by Gatsby Jewellery Ltd. Sold pieces stay on Sellingantiques as a record of what has passed through, and the next one is rarely far away.
Gatsby Jewellery Ltd has dated this piece to the 1940s.
Six rose cut diamonds (three at either end) are set in platinum at the top and bottom on the ring, the whiteness of the stones in striking contrast to the onyx. On a slim 18k yellow gold shank which splits three ways at the shoulders.

Cut – Transitional cut diamond in the centre with rose cut diamonds at the top and bottom.
Colour – The diamonds are G to H.
Clarity – The diamond are SI1 to I1; all are clean to the naked eye.
Carat – The centre stone is approximately 0.12 carat, with a further approximate 0.30 carat of rose cut diamonds.

Circa – 1940.
Size – M 1/2 (UK) or 6.25 (US). Complimentary sizing is available, can be sized up or down.
Measurements – The head of the ring is 21 x 12.5mm and sits 5.1mm from the finger.
Metal – 18k yellow gold. The diamonds are set in platinum. Tested.
https://www.gatsbyjewellery.co.uk/product/art-deco-0-28-carat-diamond-onyx-marquise-ring-circa-1940/
Gatsby Jewellery Ltd has dated this piece to the 1940s.
Gatsby jewellery are a husband and wife team who have years of knowledge and experience in dealing in fine antique jewellery.
You can view the stock and full dealer information on the Gatsby Jewellery Ltd Profile Page