Interesting - when crystals first turned up I probably tested about four or five different batches and none of them fizzed... at all. That was over two years ago... close to three. This last batch (from early May) is the first time I've seen mdma crysal fizz and it was definitely very, very weak compared to the stuff from way back that didn't fizz - the experience was definitely closer to piperazine (or similar) than mdma. It kept me awake and never really peaked, certainly no empathy or a sense of well-being. At best it felt like there was the minutest 'hint' of mdma in there, if that.
You're right, pips don't fizz but I did test a dud pill that fizzed and that's why I mentioned it. What it was, I do not know - but it didn't go black at all, just like pips = no reaction. I wish I could remember the stamp but I honestly can't.
I will say this - from experience testing around 20 to 30 'good' pills, none of them ever fizzed which leads me to believe the crystals are cut with something else - or badly cooked up.