When it comes to diamond sunglasses, it may refer to 3 things:

1, sunglasses for diamond shape faces

2, sunglasses with diamond shape frame, most of which are rhombus, hexagon, septagon or even octagon shapes.

3, sunglasses made of diamond alike materials, such as rhinestones, cyrstals and etc.

For sure, if you're searching for diamond sunglasses, you're more likely to find a pair of sun glasses with some parts that resemble diamonds, reflecting lights by their multiple surfaces.

They aren't made of authentic diamonds, which cost much, but they have something in common with diamonds, with the surface cut into multiple faces and angles, which allow lights to reflect in different directions, sparkling like diamonds.

The advantage it brings is that they cost much less than genuine diamonds but achieve almost the same bling effect.