What does crying obsidian do in Minecraft?
Crying Obsidian is a type of obsidian block. This block cannot be crafted or destroyed in survival mode, so its existance is maybe not what the questioner means. The only way to get one in Survival Mode without commands is by mining an existing one while Mining Level 0 (Stone).
You would need at least four blocks of air for the set command as well as some kind of roofing material above it because if you don’t Minecraft will remove all light levels 4 from any placed blocks that are either taller than your head or only one block high.
In creative mode, it is a small step from level 0 to level -1 and that’s enough to see the obsidian, but it’s a long way from level -1 to level 0.
In Survival mode, you have the possibility to use the map editor in order to place three blocks of your choosing, or just destroy one and then mine that one obsidian block to get a crying obsidian.