Here's why:

* Single-celled: They are microscopic and consist of only one cell.

* Rigid silica cell walls: Their cell walls are made of silica, a glass-like substance, which gives them a distinct, hard structure.

* Many different shapes: Diatoms come in a wide variety of intricate and beautiful shapes, from round to elongated, star-shaped, and even boat-like.

Diatoms are an important part of the marine ecosystem, serving as a primary producer of oxygen and food for many other organisms.