Treasure of the Pirates. 4838 Rugby Ave., Bethesda. 656-3500.

Most people like jewelry, but they don't stop to think that it's beneath their feet," says Bettye Duke, making the metamorphosis from ordinary rock to jewel sound so simple, as if anyone can transform a stone into a gem. And, the Duke family insists, anyone can . Judging from the size of their lapidary store and workshop, Treasure of the Pirates, the ranks of lapidary enthuasiasts are growing by leaps and bounds. In the Store's back room amateurs are introduced to the machines and tools with which they'll learn to grind and polish rough 'rocks" into stones fit to wear as jewelry. During each of six, two-hour sessions [cost: $45] students work with a different stone from the basic and most manageable, rhodonite, to jade. Advanced students [eight two hour sessions cost $60] practice combining stones and grinding them into more intricate shapes. Serious jewelry students can proceed with silversmithing, goldsmithing, wirewrapping and casting classes. The rest of the store is a treasure trove of exotic finished jewelry from around the world, collections of natural mineral specimens, polished stones and settings for crafting your own jewelry pieces. Books on every facet of lapidary work are also available. CAPTION: Picture, no Cation, By Rhoa Baer