Breaking news, everyone. I can definitively state that the cutting agent that's been used in meth lately forms cocrystals with the real thing. This effect is not rare or exotic, it's quite common for substances that have very similar crystal structures and molecular sizes. This is bad news. It means that it might not be possible to separate them using recrystallization. To put it in layman's terms, if this bullshit filler was a supervillain, this is like learning that he can't be harmed by bullets. Sure, he may not be totally invincible, but you just lost your most reliable, go-to weapon that you'd typically use against him. I also learned that the Marquis test for methamphetamine stops working after leaving it exposed to the air for too long, even though the test will continue to work for other substances. (Almost certainly because of moisture absorption) The Marquis test possibly cannot be used on this filler, then, whether because of the cocrystallization, or because it has a similar reaction. I didn't have a pure enough sample of the real stuff to test, once I controlled for Marquis test moisture, so I was unable to make detailed observations.
The good news though, is that the other tests I previously described do still work, for the most part. Dissolving in an alcohol/water mixture and then evaporating in a shallow dish will leave behind big, flat, prismatic crystals if it's fake or heavily cut. Small crusty crystals if it's real. The alcohol mixture that worked best was 90% isopropanol, which is available in most drugstores, and is what I used almost exclusively. Other mixtures will evaporate at different rates, changing the properties of the solvent as it evaporates.
This doesn't happen with 90% isopropanol, because it has an azeotrope with water at 85%, and at 90%, it will evaporate consistently as the azeotrope, with enough extra alcohol to form an azeotrope with any water that condenses into the mixture from the air. Higher concentrations don't dissolve either substance nearly as well. Lower concentrations, like 70%, will evaporate at the 85% azeotrope until there's only water left, depositing crystals differently at every point. The crystal structures would still look much different for real and fake drugs, but it's much more pronounced when using 90% isopropanol. The next best thing is actually just recrystallizing from water, because this fake stuff forms crystals so easily that the solution will actually lock itself under a layer of crystal, preventing itself from evaporating more. This is mostly only a problem if you need it all to evaporate so you can do more tests on it, like I did. Suffice to say, that is not something that the real stuff does when it's poured into a dish 5 mm deep.
The other test that should still work, is the melting point test. Methamphetamine hydrochloride has a melting point around 175˚C, and if it's cut with something having similar properties, that melting point will be lower and less pronounced. This is most likely the cause of the reported "crackback" effect, combined with a softer, less ordered crystalline structure.
If the filler really is isopropylbenzylamine, or any benzylamine, then the benzyl group should be removable as toluene by lithium aluminum hydride, when the mixture's free base is dissolved in ether. Isopropylamine would account for the remainder, and would be extremely simple to separate from methamphetamine, which would be unchanged. I may attempt this at some point, but haven't yet.
TL;DR: The Marquis test alone cannot detect whatever the filler is in meth lately. However, it can be detected by its ability to form huge crystals much more quickly than real drugs, as well as by its effect on the melting point of the mixture. As of yet, I know of no reliable way to separate the two, since they apparently form cocrystals. In any case, you should continue to assume that massive crystals mean massive turds in your drugs, especially if they're very clear and have smooth, flat faces.
Fuck. Knowledge bomb. Thanks brother. Who's crys is cut with this stuff, or suspected to be cut?