What can I use to repair a beach glass pendant?
Best way will be to use resin and mix it with some pigment. Some resellers even advertise their resin colors and pigments saying that they produce an opaque look similar to sea or beach glass.

Basically any pigment of the color of the pendent mixed with some white pigment will work. However some trials are necessary to obtain a coloring matching the pendent.

In the end sanding the surface of the colored epoxy with a very fine grit sandpaper will also provide the surface structure of sea glass.