A lens with two concave surfaces is biconcave (or just concave).

Gouges are most often used in creating concave surfaces.

The bed even had a concave surface, though there was no restraint web.

They are used on the periphery only and therefore produce a slightly concave surface (hollow ground) on the part.

The piece of conglomerate he had been holding had a smooth, concave surface.

A second method is drawing, executed on a slightly concave surface that subtly suggests mass.

The bottom half had been inverted and its concave surface used as a pan to cook heroin and water.

The rims were wooden, and had a concave surface for receiving the tube.

The claypan is a slightly concave surface approximately 100m across.

But keep in mind that joints with concave surfaces shed water best and are easiest to make.