When I first started on the crystal ball, I came up with lots of crazy ideas of floating skulls and fire and stuff. In the end, I went simple. I am really happy with how it turned out. Unfortunately, it is based on a "Mystic Lite" I bought from Spencers and that product appears to have dropped off the face of the earth. I couldn't find one for sale even on Ebay so you are on your own here. You can still use the basic principles, but will have to find something else for the actual ball.
So I bought this "Mystic Lite" crystal ball on sale for $15 at Spencers one day and promptly forgot about it. It is a glass ball filled with a blue opalescent liquid with a simple penlight inside. There is a motor in the ball that swirls the liquid around for a pretty cool effect. As I was struggling with my crystal ball ideas, I remembered this ball and pulled it out. As a stand alone prop, it could pass. I wanted something a bit more flashy, though. I looked at it a bit closer and, to my amazement, I could easily remove the lamp and the motor was still functional. This meant I could put any light I wanted in there.
So, I experimented with different LEDs and found that blue and green LEDs lit the ball just fine. I had just started playing with LEDs at the time so I had a strictly "on/off" approach. Today I would have made things pulsate. Anyway, I found out that a 3mm green LED inside the ball looked like a green entity floating in a swirling mass. I got to use that! So, I made a light tree from a wooden dowl and mixture of blue and gree ultra-bright LEDs (allelectronics.com). I made it narrow enough that it could fit inside the hole in the base of the ball. At the end of the tree was a 3mm ultra-bright green LED bent at a 90 degree angle. This would give me the "green entity" effect I mentioned earlier.
Now, I could have just put this on a motor and twirled the tree around, but I wanted the green entity to swim back and forth. So, I capitalized on the FCG mechanism and came up with the following scheme.
I attached the tree to a wooden spool and put the spool on a swivel (ie: a small nail hammered into a board). I attached a 4 RPM motor I got from cheaplights.com and made a cheap FCG mechanism using a small piece of wood, a large metal washer and a hex bolt. I tied two strings to opposite ends of the washer. Now, as the piece of wood spins around, the mechanism alternately pulls on one of the strings. The strings are attached to separate areas of the wooden spool. You can see in the picture that as one string is pulled, the spool will rotate clockwise. But, as the other string is being pulled, the spool will reverse direction and rotate counterclockwise. Sorry, I don't have a video. Here is another view.
The setup is pretty simple but you will have to experiment with the tension in the string to get it to work properly.
Now, I had a ball, but it was a bit small. So, I decided to put it in the mouth of a dragon skull. I had this skull I got from a New Age shop. It was a single piece so not much good to me like that. So, I made a mold of it using liquid latex and then made two duplicates with 6 pound polyurethane foam, the same way I made the fortune teller's head . I could then carve out a top and bottom jaw so that I had a dragon skull that could open its mouth. I had to cut away most of the bottom jaw so that the square base of the Mystic Lite and the LED light tree setup could be hidden down under the table.
I found some perfect-sized glass gems at Hobby Lobby and embedded two red LEDs just behind the eyes to make the gems glow red. My older sister came down to help and she painted it and added some tusks (using Sculpey plastic clay). Here is the final crystal ball. You can se the hole in the table and the bottom jaw where the base of the light is hidden.
And here is the ball again with the proper lighting...