Hehe, i am not sure about an amphetamine "guru"; maybe the chemistry of amphetamines guru might be a bit better.
I am after all one of the people who has advocated the use of dexamphetamines over the last few years, much to my fellow Bluelighters' disgust!

In any event Shulgin deserves guru status, im a hack in comparison.
Cowboy Mac is right - it could be a lot of things, but i think it depends on what you are starting with. You said ice/shabu? that indicates to me the brown bits could be:
Impurities - unlikely for proper "ice as in meth".
Dirt - the dealer is clumsy - he dropped it on the floor, scooped it back in bag with crumbs, dirt etc.
Meth itself - from an alternate source. I.E: the DD decided to mix his very good "ice" in with poorly produced "speed" (brown) to get rid of it - hence brown bits in white powder.
HOWEVER if its not "ice" at all then it could actually be that the brown bits are the potent parts.
Generally "base" or the paste style methamphetamine in all its impure, messy, glory is always beige to brown. Its UGLY - the less cut down it is, the browner it is - it also has the tendency to clump together. So in the reverse of the "ice" example, u have the brown bits as the meth thats still clumped and the whiter bits cutting agent like glucose mixed with some of the meth.
Of course from what you describe the 2nd and 3rd examples are likely; real decent ice would not have any VISUAL impurities like you describe. (but have u got real decent "ice" - who knows)
Thus my conclusion is mixed in less pure meth the DD had lying around, or dirt-style impurities that have accumulated FOLLOWING synthesis.
Additionally no DD is dumb enough to intentionally add inert impurities that is brown to something that in its concentrated form (ice) is white. If you are going to cut down "ice" you cut it down with something WHITE!
(oh is the sample you have in powder or rock/"ice" form? that may help)
I just realised i havent really answered the question just discussed the innumerable possibilities.
as i said, a hack.
