The Wham-O magic window was a toy mass-produced by Wham-O from 1973 to 1979. It was made from two 30x30 cm oval plates of heavy clear plastic, with a narrow channel between them containing microdium crystals of varying colors. These crystals are often wrongly referred to as sand.
The Magic Windows were so named because each window contained both colored microdium crystals and white microdium crystals which didn't mix, but instead created layered patterns. The window could be turned, tapped or shaken to create unique designs. The Magic Window was marketed and described as allowing the user to 'create moving changing pictures.'
The original model contained blue crystals (with hints of black crystals) and white crystals. The other colors were green/white and pink/white -- both glow-in-the-dark models. There was also a limited-run "Crystal Fantasy" Magic Window produced in 1979, which contained the blue and white, glow-in-the-dark green, plus pink crystals and tiny glitter flakes. The colored crystals had a higher density (were heavier), while the white crystals were similar to powdered silicon.
The concept behind the Magic Window toy came from inventor Roy L. Cloutier, who had a degree in Engineering Physics from Michigan Tech. Cloutier patented the concept in 1972, describing it as "An apparatus for visually presenting artistic flow patterns, comprising a container providing an inner chamber having a transparent wall for viewing the contents of the chamber, and a flowable non-liquid particulate material contained loosely in said chamber and capable of solid flow." If you'd like more specific information on the techniques involved, please visit the original patent site here.
The current owners of the original patent have made a slightly modified and improved version of the Magic Window available, created and marketed without Wham-O's involvement.