This is an elegant diamond and pearl ring from the 1920s.  It is set with an old European cut diamond, 0.94ct approx., that is assessed as J/K colour and VS clarity. There is also a round cultured pearl that measures approx. 5.83mm diameter and is assessed as ivory with greenish hue.
 
The band is not hallmarked, but it reacts to testing as 18ct yellow gold and platinum.  

It will fit a finger size K (UK/Australia); 5 1/8 (US/Canada).  It is possible that it can be resized to your needs.