I dunno, I'm kinda on the the other side of the fence when it comes to auras. I really don't liken them to any other form of divination because from the work I've done and the work of other people, there are similarities that cannot be up for interpretation. At a workshop I once went to on auras there was a staggering agreement amongst those who could 'see' and there are colors/forms that are almost universally reconized. I honestly don't understand the statement that hundreds of interpretations can be made from one person's aura; it makes me think that 99% of those people just cannot see. I feel like this because when I try to see auras, I can, and its not like reading a tarot spread or wondering what a dream means; its usually a definative color or mix of colors in a swirling gradient. Any deviations like black smudges, white flares, etc. are just...there. I'm not interpreting anything, I'm telling the color or pattern I see. I'm not saying my sight is the be all and end all of auras, but how can I not fight something when I can truly see it, like ink on a page, its there and its real to me. I'm also under the impression that free-basing the colors one sees and interpretating them with only hunches is a way of underminding all the study done in the field. Sure, someone without any training or knowledge can still see, but they don't know what they are seeing and thusly can draw uneducated conclusions that equate to 'personal interpretations'. I'm kinda passionate about auras because, to me, they are completely undeniable. And as a consumate cold reader, they are something I just cannot explain away.
But, to answer the original question to the best of my ability, I'd say an aura that looks like glass, frosted or not, would fall into the 'crystal' catagory. But then again, its really difficult to say anything about the colors that someone else saw; that's where the interpretation comes in. One person's light blue is another's pale aqua if you catch my meaning.
"Will minus intellect constitutes vulgarity." -Arthur Schopenhauer