Whisky stones are small soapstone cubes that you use to keep your grown-up beverages chilled without diluting them. If you prefer the DIY approach, they're easy to make.
The key element is the soapstone, which Instructables user alaskantomboy notes is easily available at eBay or from hardware stores. All you need to do is measure out your cubes, mark them with a sharpie, and start cutting.
Alaskantomboy used a metal cutting blade on a standing chop saw to do the trick, but also notes that if you don't have one, it's soft enough to cut with a hacksaw, though you'll have to go a little slower. Once you're finished, use some sandpaper to smooth out the sides and round off the edges (so you don't break your glasses when you drop the stones in).
Whiskey Stones from Soapstone [Instructables]