They used StazOn jet black ink to stamp the images. StazOn is a solvent ink that can be used on any surfaces, so be sure to use a good stamp cleaner. I bought the StazOn All Purpose Cleaner which is I think what they used.
The ceramic ornaments were purchased at Michael's for $1 each. The surface is flat and glazed.
This one is using a one of my mom's new It's Snow Time, Stampin Up stamps. The picture at the top of the post is also from that stamp set.
You have to be careful stamping these because the surface is very slippery. Apparently they had several goof ups, but they wipe clean as long as you do it right away.
After stamping the image, wait at least 5 minute for the ink to dry completely. They use colored Sharpie markers to color in the the images. Then wait a bit longer and spray with a sealer.
They made several different ones. The ornaments were available in stars, circles, snowflakes and trees.