British Registration Marks and Marks within a Circle by: Yvonne
the ring does not look to be British made judging from photo which is not that clear, registration marks are interesting because they tell you the date the piece was registered.
"from 1842 - 1867 the mark will have the shape of a diamond and a small circle will be directly above this mark..."
The circle should have the Roman Numeral - i (capital) within the circle. After 1867 there will be no circle and the information will be contained within the diamond.
Designers marks, for example, Carlo Giuliano was renowned for stunning enameled revival pieces and used initials within a circle.
Theodore Fahrner (1868 - 1928) was a german jewelry manufacturer of low carat gold and silver who also uses initials within a circle.
Hattie Carnegie had three trademarks and one was C within a circle. Hattie Carnegie was clothing and costume jewelry designer (1918 - 1970).
To work out the trademark we need much more information than this and clear photos from different angles. There is also the possibility of the jewelry being made by a less well known maker and very difficult to determine indeed.