Bath Window Sill

I have a bathroom with a window in the exterior shower wall. I have to wrap the window with something and have a sill (inside the shower enclosure) Years ago I think in Fl. I saw that looked like marble but were made of a plastic of some sort they came in 4" 6" and 8" X up to 8' long 3/4" thick. You can cut them with a skill saw. Does anyone know what the name of the are?
Or what is the best thing to finish the tub inclosure around the window that will be waterproof and still look nice?
The enclosure comes about half way up the window.
Thanks Lee

Can you post a picture of what you have?

No pic. it's just a window in the exterior wall of a shower and the surround comes about half way up the window.

I've done some with cultured marble before. I like to use vinyl windows that have tempered glass for shower enclosure usage. I can have the pieces made to size, and they cut with an abrasive blade on a circular saw.

It has a vinyl window but wood around the inside of it, or should I say rotted wood. I am going to remodel the bath room and want to make the shower area water proof.

+1 on using tile or stone.

If you want the wood look, be sure to use PVC trim, jambs, etc or it will ultimately rot out again.

Composite window sills