That's a good point. Glass and granite are both about 6-7 on the Mohs hardness scale. Even so, getting to a water's edge is still a long part of the journey. I'd love to see a timelapse of an individual stone. As an aside, I had another thought about copper replacement agates: They are comparably much more rare than diamonds, but far less expensive. It would be hilarious to buy every one that ever went for sale. You could probably corner the market for less than $10k.
My mother is big on rutilated quartz. Me? I've always had a thing for bismuth, in no small part because it looks like the kind of mineral Syd Mead would draw but also because the only place you find it this pure is inside giant coal-fired chimneys from the dawn of the industrial era. 
Ok. I am going to take a photo of the favorite rock I've found and post it tomorrow.