I was recently asked about how to use Obsidian in a PM, and decided to do some of my own tests (oh!! my Obsidians are fewer now!!!) and answer in the Community Forum.
it is best to use Obsidian on a Legend Item. when rerolling affix stats on a Legend Item, the values normally wont go below 75% of the affixes max value, which makes it easier to get the higher or max values. non Legend items, even with +25% Item Quality, will give you a random number from the minimum value to the highest, so you would have to be really lucky to get a max value without spending lots of Crystals.
as long as there is at least one space on an item, you can put a Crystal affix on it.
there can only be one Crystal affix on each item.
Obsidian can’t be used on Eternal items or any Pet.
if an item has a Normal (Yellow) or Epic (Orange) affix, or a Myth Stone affix that is the same as the one you are trying to get with Obsidian, it wont show up. so if I wanted Crystal Attack Speed, I wouldn’t get it if I have Epic Attack Speed or Attack Speed from a Myth Stone. I could put Crystal Attack Speed on an item, and later put an Attack Speed Myth Stone in a Socket on the same item.
Tests: placing Obsidian on items…
Level 85 item with 0 affix: 5 choices.
Level 85 item with 5 affixes: 4 choices.
Level 85 Legend item with 5 affixes (one is Set affix): 5 choices.
Level 100 item with 0 affix: 5 choices.
Level 100 item with 5 affixes: 4 choices.
Level 100 Legend item with 5 affixes (no Set affix): 5 choices.
Level 100 Legend item with 5 affixes (1 Set affix & 1 Set Bonus affix): 4 choices.
Level 100 Legend item with 5 affixes (with 1 Set affix): 5 choices.
Level 100 Legend item with 1 affix (I think this has to be a Legend or Set affix, or the item becomes a non Legend item. this happened to me once when I was a Noob, but I haven’t really tested it again to see if it would happen again): it has been awhile since I used an Obsidian on a Legend item stripped down to 1 affix, but if I remember correctly, it was 5 choices.
I didn’t do any tests on any item with 2-4 affixes, as I didn’t want to blow a lot of Obsidian on tests. it looks like the number of choices you get is conditional on what is on the item and what the item is.