sorry, can't help with suggestions of purpose, although simply decorative might be an option. No need to assume 'cut glass', it appears very obvious
The word crystal is often coupled with 'lead', and even though this may not produce a ring (glass possibly too thick or wrong shape), it may well still contain some lead to assist with cutting. If this were from some time earlier in the C20 it might be a cigarette holder, toothpick holder. What is the extent of wear, if any, and is the cutting sharp?