The photos from that cave are incredible.
A bit easier to find is a type of igneous rock called a pegmatite which has a similar composition to granite but has much larger crystals. They form from the last part of granitic magma when magma contains few crystallisation nuclei and is saturated in water and fluxing agents such as chlorine, fluorine and boron that become more and more concentrated in the residual melt.
So the crystals, often of unusual minerals like beryl and tourmaline, grow incredibly slowly and can become enormous - the largest single crystal in the world is a beryl something like 18m long and weighing more than 350 tonnes found in Madagascar. There are also some massive pegmatites in the Appalachians with individual feldspars weighing more than 200 tonnes!
I used to have some lovely pieces of pegmatite containing beautiful black tourmaline (schorl), massive sheets of silver lithium micas like phlogopite and perfect pale green apatite I collected from a landslide at Megilligar Rocks in West Cornwall - I wonder where that ended up in all my moves over the years?
