English: The black patterns are formed by grains of a mineral called magnetite, an iron oxide. it is a normal part of most sand. When waves wash the sand, the iron, which is heavier than grains composed of other minerals, resists the movement of the water slightly better, and collects in a thin layer on top. As can be seen in this images, the slightest obstacle produces a tapering pattern. This is taken in Costa Rica, not that it matters....