This version is a bit pricy, but very well made and shows the phenomena of spontaneous stratification better than any other versions I've seen:
From Amazon: BUY NOW
Kinetic Sandscape: the phenomenon of spontaneous stratification is a striking feature of this sand motion desk toy. Typically pouring stuff together results in further mixing, yet here, when the sand mixture descends through a gap in the air bubbles, the resulting pile is ordered into sorted layers. The larger rounded dark grains separate from the smaller sharp-edged white grains forming the layers you see here. The darker grains alone would stack into a steeper pile (larger angle of repose) than the white which would form a less steep pile. The stratification process is a slow one in this device, here about 30 minutes have been condensed to the 60 second IG format.