A loose piece of calcite. Shiny!
If you're not severely claustrophobic, I can warmly recommend the Charleston Gloworm Cave Tour in New Zealand. The two of us went in with our hiking shoes, jackets, cameras, helmets with headlamps and trusty guide. It's as close to a cave exploration experience as this clumsy tourist is likely to get.
Of course, we followed the Singapore airport butterfly garden motto: Take nothing but photos, leave nothing but footprints.