Vintage Modernist Brutalist silver ring. Size 6.5

Vintage silver designer ring. Modernist Brutalist Mid-Century Modern (MCM) style. Stamped 925 and has a mark SF. I was told that it may indicate that this ring is silver filled and not 925 sterling silver. Not sure how to confirm that, so selling as is. In excellent condition. Size 6.5. The princess cut stone looks like either a crystal, or a rhinestone. Unisex, so will be listed in both men and women categories.