Question: what type of crystal is this 'special crystal' you mentioned? what is special about it?

Andrew McKinley answered on 17 Jun 2010:

The crystal is something called ‘vaterite’ – it’s a very special form of calcium carbonate (the same stuff that’s in our bones, limestone and shells – and many other things). What makes it special is that it grows in spheres – You’ve probably come across many crystals that grow in cubes, in hexagon-shapes, pointy needles etc., but probably none that grow as spheres. These spheres though are really small – about 10 millionths of a metre across – ten of them side-by-side would be the same thickness as your hair!
Another thing that makes them special is that when you shine polarised light on them, they start to rotate, meaning that you can effectively stir tiny volumes of liquid!