When I was younger, a friend had a toy that I loved but was far too nice to steal. It was, for lack of a better term, an oil slick trapped in plastic. It was flat, with one side a clear plastic and the other side some sort of sticker. Inbetween the sticker and the clear plastic, in some sort of very slightly bubbled area was, I assume, some mixture of oil and water. If you pushed the bubble down or rolled your finger across it on the sticker side, the plastic side would show you constantly changing colors like looking at an oil slick on water. I presume the "sticky" side of the sticker was silvered in order to help light reflect and bring out the colors, but I'm not sure.

This was about 25 years ago, so my memory isn't perfect. I have no pictures (I Googled for a variety of terms including oil/rainbow/slick/toy/flat/plastic in order to find this, with no luck), but I remember this being decent sized, about 4"x4" or larger (being flat, its depth was negligible.) Does anyone know where I might get this, or something like it, or, failing that, how one would go about making this on their own? Any help would be appreciated!