The Harmonic Ruffle starts as the inside half of a torus. Then we apply a ruffle function (a variation on the one used in the Equation To Object tutorial) that's a combination of two sine waves at two different frequencies.
Harmonic Ruffle 02 is colored by Gaussian curvature using a repeating color map. It's available in Full Color Sandstone.
The toolkit we use to turn these equations into printable objects is called
Anethole, and is available on GitHub.