Might sound patronising, but are you smoking it efficiently? If the flame is too close it will start bubbling and slowly go a shade of brown, varying very light brown (normal residual stain) to a thick blacky brown caking of residue. As i understand it, this is because the meth is burning instead of vapourising. My meth is fine, except when i hold the flame to close, which produces a slightly darker liuqid and an unpleasant taste. By the, way, at face value the crystals appear almost flawless, with crystal clear, well formed shards. No noticeable impurites.
But im not a chemist, so if anyone else has a more factual theory, please prove me wrong...