Locality: Rainbow Caterpillar Mine, Haris range, Northern Territory, Australia
The locality is particularly important on this one, because it only comes from one place. Hopefully the photograph speaks for itself. This is one of the coolest, prettiest things I have ever seen.
The rock is a combination of 75% orthoclase (K-rich feldspar) and 25% albite (the Na rich end member of the plagioclase feldspar family); this basically just means that it has come from a granite. During slow cooling, certain materials undergo solid state reactions, forming more stable configurations. These are called ex-solution lamellae
This rock is 25% albite and 75% orthoclase (the more nerdy among you