You convert material of one sort into a product that is of the same material.
I'd say that sand and glass are not the same material. Common glass is only about 75% sillica (the main component of sand) . While I suppose it's possible that the rest happens to be mixed in with your sand, I think it's a stretch.
Fabricate on sand could help you make a life-sized sandcastle, but you wouldn't be able to install glass windows.
If you mix together white sand and some of the additives used to make glass, I don't have any problem with you using fabricate to turn them into glass. You can't just go to a desert or beach and turn the surface into glass, though; you need the additives.
'Common glass' as we have it now is not common in a fantasy setting. I think you would get what we would consider crappy glass.
To take this to it's ridiculous conclusion, would a lich that spent 100 years developing the science of molecular chemistry be able to turn coal into diamonds?
No, because gems are something of high quality, and there is no craft in gem making for him to use in the spell. A wizard swordsmith knows how to assemble iron and steel into a masterwork sword, a lich has no idea how to move carbon atoms into the precise shape of a diamond.