Help with identifying mark on silver ring

[Sweetxs]

Hello! I just bought a vintage silver ring that has a mark that I've never seen before, and I didn't manage to find anything about it online. Do you have any idea what the "SIX 925" mark means? The stone seems quite special, I looked at it under a microscope and I think chances are it's either a diamond or a similar gem, such as white sapphire or moissanite.

Appreciate any information any of you guys might give me on this!