No, these are not a new variety of Liquorice Allsorts but the silicon dioxide shells that encase diatoms, microscopic floating algae that flourish in lakes, rivers, oceans, soil and even in the loo. There they lurk in the circle of gunge attached to the wall of your cistern. Diatoms first appeared 200,000 million years ago and became so prolific that deposits of their shells measure up to 900m deep. This diatomaceous earth is used commercially fr filters, as insulation material and as an abrasive.
The diatom's hard glassy shell, which fits like a pill box, does not stop it from being devoured by pond snails and other small animals. If you view some of the 5,000 species under a microscope, you enter a luminous world of different shapes, sizes and movement.