When you get information about what is allowed and not allowed when exploring Crystal Cave in Sequoia National Park (CA), one point that will be repeated is that you’re not allowed to touch the cave formations within Crystal Cave. Touching the cave formation damages them. When you touch any formation, oils from your hands are transferred to the formations which could inhibit their continued growth. Crystal Cave is a living cave and is still actively growing.
Since there’s certainly a desire by many people to touch the cave formations, there is a Touching Rock at the cave entrance which you can see and touch. This hopefully gets the touching urge out of your system (or at least allows you to resist while in the cave). Millions of people who have entered Crystal Cave have done so by first putting their hands on the Touching Rock: