We've seen recently that a lot of Chinese sellers are popping up and selling fake K2. This is not a fake in terms of being a fake crystal, it is a genuine crystal, but one that has been mislabelled as a more expensive one so they can charge more money for it (like the very common practice of selling Blue Calcite as Celestite). What they are selling as K2 is in fact another lovely blue spotty crystal, Blue Apatite in Cleavelandite, however it is from Madagascar, not Pakistan, which totally rules it out of being K2. This is what genuine K2 looks like, and the Madagascar Blue Apatite for comparison:
K2 can only be found in Pakistan, named after the second highest mountain in the world, K2, where it is found in an incredibly remote area. Despite it's slightly odd appearance that makes it appear dyed, it is made up of Azurite in Granite, a very rare geological combination in which are the blue spots over the opaque granite background. The very fine grained granite is composed of quartz, muscovite, biotite (both mica minerals) and sodium plagioclase (a type of feldspar). You also sometimes see spots of green Malachite in with the blue Azurite as well!
The Granite is totally opaque, with a solid look and black spots, whereas the Apatite in Cleavelandite has a much more soft looking, translucent base. The spots are often a similar shade of blue, although the Azurite tends to be much more defined. Below are two images of Chinese sellers passing off the Apatite in Cleavelandite as K2: