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A model boat floating on sulfur hexafluoride (gas significantly denser than air) at the Physikshow of the University of Bonn!

This is a classic chemical reaction. It's called the iodine clock reaction. There are several variations of how this chemical reaction can be performed using different chemicals than the ones I used in the video. You can order clock reaction kits from several science related websites. You can also use simple store bought chemicals like vitamin C, iodine, hydrogen peroxide and starch. A quick internet search will turn up multiple ways of performing the experiment.

Gemesis diamonds are mostly yellow due to the Nitrogen in the atmosphere getting inside the crystal during the growing process. Apollo diamonds can be created in all colors including colorless depending on how much impurity is introduced in the growing chamber. e.g. add boron to make blue diamonds.