Gatsby Jewellery Ltd has dated this piece to the 1920s.
Accompanied by an independent jewellery report from IGR (#30308561).

Cut – Old European cut.
Colour – H.
Clarity – SI2.
Carat – Approximately 0.40.
Circa – The diamond was cut and polished in the early part of the 20th century, in newer setting.
Measurements – The pendant, including the rubover setting is approximately 5.65mm in diameter. On a 16 inch trace chain.
Metal – Stamped ‘750’ indicating 18k gold, and tests as such.
Gatsby Jewellery Ltd has dated this piece to the 1920s.
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