The Crystal Skull, page 1

Granted it wouldn't be "glass" or high grade glass or sort, but at the time the technology was present to heat the material to a thousand degrees at the least. Granted metal working was nearly void in the region, due to a lack of iron, but I should think this would mean they'd focus heat on other objects, such as stone, which the European//Meditteranean cultures did not pick up on as a mastery, until much later. Posted by 5POF

Quartz begins to melt at around 500-600 degrees C: this is a very high temp to achieve given primative methods, but I will agree that it would be possible with a properly constructed kiln with a proper bellows system. However, I have a hard time believing that skulls of this type could be made like this, without some kind of contaminant entering from the furnace. (again, something that could easily be settled by chemical analysis of the crystal)

Also, even if it were largely cast in a molten state, (which would produce glass, not quartz) I still find it hard to believe that some form of tool mark from finishing would not be evident on it somewhere.

Also would it be possible to simply heat it to a degree that allows a shaping of it, such as the eye sockets, but perhaps not hot enough to completely lose its crystaline structure. Or maybe even a way of heating and rapid cooling, tempering the stone as you would steel, for when you temper steel you are strengthening its crystaline structure are you not? Posted by 5POF

I honestly have never performed any kind of experiment to find if it is possible to partially melt a quartz crystal and still retain its crystal structure. My first thought is that it would be difficult or impossible. Keep in mind, quartz is a very stiff material, and unless it was completely heated through all of its mass at a uniform rate, any areas of differential heating will result in differential expansion, resulting in the entire crystal shattering. (Ever put a non-pyrex glass dish in the oven?)

To answer your question about letting it slowly cool, that is what sets up Quartz crystal structure. Glass (amorphous or non crystalline quartz) is what you get when you melt quartz and "quench" it or cool it very quickly, so fast that crystals dont have time to form. The slower you cool it, the better, larger, and more coherently the crystals will form. Therefore, if you melted it and allowed it to cool very slowly, you would only have more crystal structures.

I am not sure that I agree with your idea that it may be a fake, as I cannot see even modern techniques making these skulls without telltale signs, and I believe they have been around since at least the turn of the century or longer.

As for your idea that they are glass that would not deform, I am not sure I agree, but will say it is within the realm of possibility. Either way, a few analysis would certainly explain a lot if we were given access to do so.