To make a seashell shape, sweep a circle around a helix while applying an exponential decrease in scale, such that successive turns are tangent to one another.
Seashell Tangent 02 has a decrease angle of pi/8, and a rise angle of 3*pi/8. It's colored by Gaussian curvature, and available in Full Color Sandstone.
The toolkit we use to turn these equations into printable objects is called
Anethole, and is available on GitHub.