All Alchemy Stones have a very durable outer clear coat to protect the painted design. It's actually similar to the sort of clear coat you'd put on wooden furniture to leave it out in the weather, year around.
There are some secondary benefits to the gloss protective top coat on all Alchemy Stones. For one, know how beautiful and bright wet stones are, when they are on the beach? And how they seem to become "dull" when you bring them home, and they have dried? The finish coat allows that "wet look" brightness of the natural stone to show... in addition to enhancing the pain colors.
In addition it means you can keep your Alchemy Stones as decoration in a place like a flower pot, or Fairy Garden outside, if you wanted to. Although the coat would eventually break down (like varnish on outdoor furniture) it would certainly protect the design for several years.