What a wonderful classic antique signet ring. Men's antique signet rings do not get much more classic than this ring here. Cannot see clealry the letters in this photo.
The crown followed by 18 is a UK gold standard mark on 18kt gold from 1798-1974. The ring is a 19th century signet ring. During the 19th-century, the signet was a mark of a gentleman everywhere.
The oval bezel is set with the Hardstone - "Heliotrope" which is a variety of chalcedony. It is also known as "bloodstone" as the very dark greenish blue against the red speckles resemble spots of blood. So the story goes:-) The red inclusions are,in fact, red jasper.
The plain and substantial settings were usually standard in type.