Reserve a spot on the Discovery Tour at Jewel Cave National Monument in South Dakota, and you'll be able to explore the cavernous target room during this 20-minute, accessible tour of a portion of Jewel Cave. According to the National Park Service, "During the Discovery Tour, you will see two types of calcite crystals commonly named nailhead spar and dogtooth spar, which are the "jewels" of Jewel Cave. You will also see manganese and paleofill, which are not cave formations, but are important to the geology of Jewel Cave.