Ok, most of my knowledge just comes from Bluelight but I'll try explain what I know:
It's not just purity that effects the appearance of the meth, it depends on factors such as how the cook made it, what processes etc and what pre cursors they used, these things can contribute to the strength, appearance, quality, form it is in.
Also smokable 'speed' in powder form will always be cut with different substances and that effects the appearance and purity.
Purity doesn't always directly relate to the appearance though, as you can definately get large clear shards from from one batch that have a very high purity, and get identical looking shards from another batch that will be less pure. I think it largely depends on the skill of the person making it. I'm pretty sure its possible to cut the meth before its crystalised also while still retaining the same clear, glassy appearance.
So theres alot of factors that can affect the quality, and while large, clear glassy looking shards will often indicate a high purity product, it isn't always the case.