The more pure the dirtier it looks...like someone stepped on it.
not necessarily but for people who have no idea it is a good way to for them to look at it.
if you are getting shards (or crystals) it can go either way because depending on the preference of what the chemist likes to do the final rinse with they will turn either a dirty white/grey or will stay a dark amber/ orangish color
ive had great and bad of powder and shard except for amber shard (always great in my experience) it seems to me the hardest to fake in my experience but also the hardest to come across
but like i said before if it is your first time buying from anyone buy a test kit it really is the only way to be 100% about what you are taking