Magnetite is a black iron oxide mineral that is common in sand form in the hills near where I live. It is easy to find in the spring, as it is denser than the surrounding sand and concentrates in the gullies when the spring melt runs through. The black sand is attracted to magnets, and that makes it easy to separate out from the rest of the material, so that it can be used as a pigment for watercolor paints.
The sandy particles mull up fairly easily, and result in a dense black granulating paint with a red iron oxide secondary color.
Here's the best part - you can manipulate the magnetite in wet paint with a magnet, causing the pigment to form fine lines in the magnetic field - and that makes for some fantastic effects on paper.