Garnet. Credit: Mineral Information Institute.
The helicoid is the second oldest known minimal surface; it was discovered by Jean-Baptiste Meusnier in 1776, thirty years after the catenoid. It is the only minimal surface, apart from the simple plane, that is also a ruled surface. The helicoid is the surface swept out by a line that always intersects a fixed axis at right angles and that rotates uniformly as its point of intersection moves uniformly along the axis. This line intersects any cylinder concentric with the axis in a helix. The helicoid has a wide variety of shapes and is a familiar sight in everyday life, taking the form of everything from spiraling parking ramps to screw threads.