In a Mexican zinc mine, gypsum crystals over 30 feet long have been found.  They need temperatures over 120F and submersion in water to grow, so it’s a rare find.  One would deduce that such conditions existed for a long time in this particular cave.

The Giant Crystal Project Site here.

More on gypsum here (a key ingredient in plaster and drywall as well as chalk, so it’s likely all around you), a brief youtube video here of its molecular structure.

Brief note:  It’s not the end of the world, National Geographic, so please just focus on the science part of your mission, and let me think for myself and maintain my cherished skepticism:  The National Geographic-Marching To The Eco-Drumbeat