Kenneth G. Libbrecht, from Caltech has some gorgeous microphotos of snowflakes at http://SnowCrystals.com. I took one of his images, removed the background, and then made it into a height map using cura's photo import.

I set the background depth to 0, but you still get one layer, so print with a .1 cutoff, or edit in netfabb or meshmixer.

The ornament version was combined with a ring I made in openscad (trivial, so I only posted the stl here) in Meshmixer.