black tar herion to good to be true?

[orgasmic]

Hey.. I been doing herion for like a year.. well anyway this time when
I went to go score some herion.. the person we usually get it
from pulled out 3 huge rocks which is about a half ounce of shit.. Now
my girlfriend doesn't know if it is real or not. Our herion comes
in a black like rock form, which is called black tar. The rocks look
crystally and break into shreads almost like it's dried out this
time. When my girl injected it, her arm started swelling up but
it wasn't enough to get her high so we are not sure if it is
real. What my question is.. How do I tell if this shit is real?
Because the person that sold it to us said he found it laying around
somewhere by the dope house, the person who has been getting it for us
this whole time (our friend). We ended up getting it for only 30
bucks. Is this to good to be true? What should I do?