Sand is not a rough mixture of various rock particles, but a granular material. The size of a sand particle ranges from approximately 0.02 mm to 2 mm. The appearance of the sand particles depends on the rock from which they were formed by erosion. Black sand is made from volcanic rock, and oatmeal sand from quartz and feldspar grains. Red sand contains a lot of iron-rich minerals, which are frequently found in sandstones, and white sand is mostly composed of small, grated seashells, snails or the chalk from coral reefs. If a rock containing gold erodes, the gold particles are freed and get stored in rivers. Such findings led to the so-called gold rush in 1848 in California.