BURIEN — Shell Hallahan wiggles a finger under a golfball-size rock and gently pries it from the wet sand.  Scanning the gravel beach of Seahurst Park in Burien during low tide on a recent sunny Friday, Hallahan was on the prowl for nubs of honey-colored agates. “They’re kind of like unicorns,” she said. When held to the sun, agates appear to glow, revealing delicate bands like tree rings.   But this one isn’t agate. Brushing it off, Hallahan still admires the striped hunk of brownish jasper. “Sometimes a pretty rock is just a pretty rock,” she said.  The search continues. If they were easy to find, it wouldn’t be as fun.  “You never know when you’re going to find a jewel,” she said.  For geology wonks and amateur collectors of minerals and fossils, Washington offers a jackpot of dazzling treasures that tell the story of Earth’s mighty history. You won’t find jewelry-grade rubies and sapphires here.…