You can start unlocking the Obsidian farm, after reaching player lvl 115 and upgrading the Grand farmhouse to lvl 10. This upgrade will require diamonds, which can be earned by participating in any of the Dream Farms events.
If you visit your Grand farmhouse lvl 10, you will notice that there are now several new tabs available. There you need to complete a research that will unlock the possibility to upgrade the Grand farmhouse to lvl 11. This research requires obsidian, which can be obtained by participating in the event Farmer's league. Once this is done, you will have access to the next and final research - "Unlock the obsidian farm", located in the second tab.
The Obsidian Farm - Unlocking Video Tutorial