On PS4 I tried to test it's obsidian gathering since it is "Rank 1", well all it was gathering was stone. So I went to another saying maybe something was wrong, NOPE it does not gather obsidian. Just use an Anklyo for that.
Doedicurus can farm obsidian and will do a 2x the rate of the anky. This depends on which map you're on. Tested on the island. Can't use on the volcano on rag (tested) will test others when I get chance believe it is map or reign dependent.