It’s a little tricky to tell from the image due to the focus being a bit skewed but my guess would be some variation of an old cut. Likely an old miner? It’s got the trademark chunky facets and wonky symmetry, and I think I spy an open cutlet, but again it’s hard to be certain since the picture is not very clear.
Is it an antique/vintage ring? because if so then I think this is a safe bet.
May we see a photo of the whole ring, the style of it would be helpful to gauge the diamonds age etc.