I'm no expert on meth, but I'll just say this.. It's not wise to make any generalization about quality, (about any drug), based solely on psychical characteristics. I run into this a lot with people asking your same question but about heroin. I've had stuff that looks great by typical standards, but was garbage, and I've had dope that looked like it should be shitty but was amazing..
Granted, I mean if it's total shit you can probably tell but.. don't assume 100% of the time that just because the product follows some rule about its psychical appearance, that's its going to be weak or strong.
well if someone handed me a bag of what looks like really high quality glass, would I buy it, or not buy it because theres no way to prove whether it's good or not? with meth, it's different than heroin, coke, etc. Meth forms solid crystals when it is relatively pure, because impurities make crystals harder to form. While, yes, this crystal could actually be a completely different chemical than meth, but assuming that it is meth, and it's on point, if you're getting large shards, then your ice is some very high quality shit. also clarity of the crystals determine purity but that would be very hard to gauge from that. I've personally never had anything that wasn't large shards, but I've seen pictures of "crank" or "shake n bake" meth, and it looks like diseased cocaine. With meth, the appearance of the substance has everything to do with it's purity/quality/etc., whereas heroin seems to come in weird forms (i've gotten paste, tar, and nasal spray [don't ask]), and the potency is entirely unpredictable. With meth, if it's crystal, it's good, no doubt. If it's chalk, it's usually low-mid quality. If it's that nasty stuff that looks like mold, that's shake n bake, and i recommend you don't partake.
You cant determine the EXACT purity of the ice, though, only if it's good or bad.
Dope will have a kind of burning sensation, and be really bitter. You obviously know how meth tastes though, as it's impossible to forget lol.
that sour-spicy-bitter burn... i almost like it.