Mid-Hudson Valley Gem & Mineral Society hosts its annual gem, fossil, mineral and jewelry show and sale for two days at Gold’s Gym. The theme of this year’s show is “The World of Crystals.” Shop from 30 dealers, watch a lapidary (gem-cutting) demonstration, and be sure to check out the rock and mineral displays set up by the Vassar Geology Museum and by MHVGMS members. Kids receive six free rocks at Glitter Mountain, and they can sluice for treasure at Garnet Gulch. High school students will love the Earth Science Scavenger Hunt.