The only gear you can craft with obsidian block is on the Offhand Weapon. In the ring example, obsidian block will only occur on Drop from the Warrior class frost raven. Warrior frost raven ring has epic block affix which can become crystal / obsidian affix on drop.
Warrior Ring with Epic block affix
Wizard Ring - no block affix
Rogue Ring - no block affix
List of obsidian affixes
Any gear with an Epic affix listed in the pool below has a chance to become crystal affix on drop. For example, Wizard and Rogue rings above have a chance for the gears to drop with Dodge obsidiam affix (both have dodge epic affix). I hope that clears things up.
HP On Hit
MP On Hit