Do you remember Magic Rocks? These were sold at dime stores and in the back of comic books. I remember buying a set at a Stuckey's on vacation to Florida one year. The toy I remember looks like the picture above. Here is a picture of an earlier Magic Rocks set:
How they work: you get liquid sodium silicate and colorful rocks (actually metallic salts). You put the rocks in a pretty glass container, and then the sodium silicate (mixed with water), and the rocks grow. I googled and this simple science toy is still around!
I always liked these ... they reminded me a bit of stalagmites (I liked caves, too!) and also a bit of those color drip candles at Italian restaurants back in the day.
Here's a little minute and a half time lapse video:
Who remembers this science toy? I'd love to hear from you in the comments, below.