Vintage 925 sterling silver designer ring, Modernist Brutalist, size 6.5

Vintage sterling silver designer ring. Modernist Brutalist Mid-Century Modern (MCM) style. Stamped 925 and has a makers mark SF. 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.