Only sea glass will have the proper sea glass look. I'm a bit anal about this because I make jewellery from the stuff and am fed up of the market being flooded with sea glass that isn't. Tumbling bottle glass for a while even in sea water does not make it sea glass, no more than sticking some bead caps marked Pandora on a cheap Chinese bead makes it a Pandora bead. Anyway, you'll get a frosted look if you tumble glass, I think someone told me it'd take a couple of weeks, but it'll just be frosted. To get the characteristic 'C'shaped frosting marks on genuine sea glass you'll have to wait 50-100 years and let nature do the work for you.
I've had the odd bit of sea glass go bang on me, one piece even refused to melt. I've not tried on my Minor, will have a go this weekend if I get a chance (my Mango bead corer comes tomorrow, I might not be able to resist).
I'll get off my soap box now
