The Himalaya Tourmaline Mine is part of the old gem-mining country east of Julian, where pegmatite veins produced pink and green tourmaline, quartz, garnet, lepidolite, and other minerals. The historic mine itself is in the Mesa Grande district; the traveller-facing dig is usually run from the Lake Henshaw area, where visitors screen ore from the mine and keep what they find.
It is a hands-on outing rather than a museum, and that is the reason to go. You will get dusty, learn quickly that gem hunting is mostly patience, and come away with a better sense of why this corner of San Diego County mattered to mineral collectors. Check operating days before driving out, as public digs are seasonal and weather can change plans.