There was once a instructable how to make one, but it was not quite good. You can still find it here: http://www.instructables.com/id/EJCIH9CHRMEYF8MQ68?ALLSTEPS
I'll try to make it quite simple and I have photos and a movie (my own even :)!
This project will cost you dearly: about about few promiles from few promiles of your monthly sallary. And will consume an enormous amount of time, like... 15 minutes.
I always loved lava lamps, but buying one was just a little bit beyond me so I've decided to make my own.
So let's go!
Please note, aside from this instructable being my first one, I'm not an native english speaker so any comments pointing out mistakes are welcome.
I'm still thinking about a stand, so the bottle will be warmed and lighted up by a light bulb.
UPDATE: the water-colours were a "not-so-quite-good" choice. See the last step. Also the brush I used to mix the paint with the water left some of it's hair.
UPDATE 2: The "dust" in the last step is not from the water colours. I was from the calcium tablet.
Also warming up with a halogen lamp fails to produce the desired lava lamp effect (the base was (about 48 deegres Celsius hot). I still don't know if it's the effect of the calcium sediment or the water-oil combination just won't work.
LAST UPDATE: The oil-water combination just dosen't work. Sorry - this is a "one tablet - one go" instructable.