This wonderful thing is a naturally formed nodule that has formed from agate crystal with some water inside.
It you hold a torch behind it (like in the video) you get a really good view of the water!
The translucent window in the top shows where the (millions of years old) water moves around inside it. It takes a moment to tune your eyes in to be able to see the water moving but once you do it's quite easy to see, it is wonderful.
The whole nodule measures about 8.5cm long x 6.5cm wide x 4cm deep.