Natural light is really the best method for photographing sea glass - however, sometimes it is fun to try out other alternatives.
A great way to illuminate sea glass - without getting glare - is to use a light table. This can be done at any time of the day and the results are pretty neat. I have an inexpensive Crayola Crayon light table, of my daughter's, that I use. To keep a continuous flow when photographing something at a side angle... I just placed a piece of white poster board behind the light table. Another thing you could do is place the light table in a white light box to get the same effect. You can get cool effects without spending a lot of money!