For the last few years, everyone has been using really poor techniques to gauge the quality of methamphetamine. "Crystal size" and "crystal clarity" especially. Those don't mean a damn thing if the crystals are of something besides drugs, which is more and more frequently becoming the case. If crystal size was a good indication of drug potency, then we'd all be hanging out here right now:
http://s.ngm.com/2008/11/crystal-giants/img/crystal-cave-615.jpg
A filler has become increasingly used that looks just like high-quality stuff, but is probably isopropylbenzylamine or similar. Not only does it form giant crystals without even trying, but those crystals test positive on some tests. They don't have any effects though. At least no good ones. I'm not 100% sure what this stuff is, since it didn't respond to the test I theorized should work for iso, so I'm just going to refer to it as "bullshit" from here, because that's what it is to you.
But most vendors and dealers won't sell pure bullshit, because all but the most naive drug users would catch on. And let's face it, most of us are quite experienced. So they play tricks. They'll take a nice, huge-ass crystal of bullshit, and drop it in a hot, saturated solution of real drugs. As the solution cools, it forms a crusty layer of goodness over a bullshit core. A chocolate-covered turd, if you will. See, they know that the first thing you're gonna do when you open that bag is take a chip off the largest crystal to sample. And you'll be quite pleased indeed with that one. The more you go, the less pleased you'll be, but that's just tolerance setting in, right? Wrong.
Now, there is a simple test you can do, taken from industry, to determine if you're getting mostly the goods, or mostly bullshit. Take that crystal, and smash it, into approximately equal-sized pieces the size of rice grains. What would help you most is a Marquis test, and I can tell you how to make one easily enough, but if you don't have one, then dissolve some in as little boiling rubbing alcohol (a mix of isopropanol and water) as possible and let it crystallize somewhere where it won't be touched, preferably on a plate or dish or something where it'll evaporate faster. This should only take 20-30 minutes. If it forms giant, completely transparent crystal structures, it's as fake as a 40-year-old stripper's boobs. If it forms big transparent crystals with crusty stuff at the edges between them, then you got a mix of the two. Small, crusty crystals are actually a GOOD thing! It takes lots of time and patience to grow big REAL crystals, and you sure as fuck couldn't do it on a plate in less than a half an hour.
Another thing about bullshit is that it fucks with the Marquis test. See, the Marquis test relies on polymerization to produce the colors it does, and bullshit interferes with that. So a mixture that is mostly bullshit will show little if any reaction to the Marquis test. Now, if you want your own Marquis test, you just need concentrated sulfuric acid and formaldehyde. If you have formaldehyde solution of 30% or so, you can just mix the two (one part formaldehyde to two parts acid), but if you don't, you can use other things too. The plastic called "acetal", or "Delrin", or "polyformaldehyde", or "polyoxymethylene" will turn into formaldehyde in the presence of acid. If you're already into chemistry, the clamps that they put on glassware to hold them together are usually made from this. It forms formaldehyde slowly in 50% sulfuric acid, but it will eventually work. "Trioxane" and "paraformaldehyde" work a lot better for that though. There's currently a lot of trioxane-based fuel tablets being sold cheap as military surplus, since they switched to a different fuel. At $8 with free shipping, you can have a lifetime supply:
http://www.ebay.com/itm/282488362332
There's usually purple stuff mixed in with it. I don't know if that purple stuff would affect a Marquis test, but if you empty all the packages out into a plastic container, the trioxane will evaporate slowly and form small, oddly-flexible crystals on the sides and top, and these are pure trioxane. You can speed up the crystal formation by putting the container on top of something warm, but it'll probably take a day or so.
Sulfuric acid can be obtained in the US as acid-based drain opener. You can spot it because it usually comes in bottles that are inside plastic bags. Home Depot and Lowes rarely carry it, but Ace Hardware and TrueValue usually do. Rooto is the best brand I know of, because it doesn't contain dye or carbon particles. If it does contain dye or carbon particles, then gradually adding hydrogen peroxide solution while stirring it should eventually clean it up. You shouldn't heat it at first, because it will heat itself very quickly when water is added to it. It goes without saying that you can easily injure yourself very badly if you are careless when doing this. After you add peroxide, you have to heat it until it starts fuming, to decompose the peroxide. Otherwise the peroxide will oxidize anything you test with it, and there will be a bunch of bubbling and no color change.
If you have a solid form of formaldehyde, heat it with equal volume of 50% sulfuric acid until it's mostly dissolved, then pour it into about another volume of concentrated sulfuric acid to make the Marquis test. If you have any of the known-pure substances that are mentioned on the Wikipedia page:
https://en.wikipedia.org/wiki/Marquis_reagent
You should test them first to make sure you made the test correctly. Aspirin is a good one to test. Adderall works as well.
Again, drug dealers count on the fact that if you get a huge crystal, you won't immediately smash it up, so if you want to ensure purity, that's exactly what you should do, and take samples from the middle. Also, sample the dust at the bottom, because nobody ever starts with that. Really, take at least five samples (they only need to be the size of a grain of sand) from different parts, and see which (if any) show reaction.
If working with sulfuric acid, you need to know what materials it can be stored in. Glass is always a safe bet. The two plastics that are compatible with it are polyethylene (has a "2" recycle number, and the letters HDPE or LDPE for "high-" or "low-" density polyethylene) and polypropylene (has the letters "PP", I can't remember the number off the top of my head). PETE (plastic soda bottles and most clear, flexible plastic) should work temporarily, but will disintegrate over the course of days or weeks, or if the acid is hot.
I've gotten sulfuric acid on my hands before, and it's not that bad as long as you get it off quickly. Lots of water very quickly is best, and having ammonia or baking soda solution handy to neutralize acid is always prudent. And definitely avoid breathing formaldehyde. You'll smell it immediately, and it's not a good smell, so at least it's easy to avoid. Not as toxic as cyanide or phosgene, but certainly something that it's better to not breathe.
As of yet, I know of no DNM vendors that sell pure product, sadly. Even those with 100% positive feedback and gushing user reviews have sold, at best, 50% real and 50% bullshit. This wasn't always the case, but has become the case over the last year or so. I've tested this stuff for several friends who have given me samples, and have observed this fake filler bullshit from every one of them in at least the last six months, so be warned.
TL;DR: Assume that from now going into the future, getting large crystals is a BAD thing. Dissolve that shit up in vodka or rubbing alcohol and let it evaporate. (Do NOT swirl it around or blow on it to make it evaporate more quickly! This breaks up crystals before they can form!) Once you've seen what crystal patterns look like of real stuff and fake stuff, it's really very simple to tell them apart. And for whatever the latest filler is, the Marquis test will give a definite answer.
edit1: When testing a sample to see crystal structure, it's important to use the same solvent every time. As such, I recommend using 90-91% isopropyl alcohol, which is available in most pharmacies. Not 99%, not 70%, 90-91%. Reason being, this mixture is very close to azeotropic, which means that crystal formation won't change as one of the solvents preferentially evaporates and concentrations drop. This is actually very important.
Whatever you've been smoking, must be good stuff!