The Block of Goo is a green jelly-like bouncy building-block.
It can be crafted in your Crafting Menu (default key "q"), but only after the according rafting-recipe has been unlocked by
- taking a Glob of Goo (an occasional loot or pet-harvest from most Creatures, including Pigsies) and
- crafting or picking up a Stone Mining Cell
To craft 8 Blocks of Goo at a time, you'll need (as of R43 in May 2017):
Blocks of Goo are fireproof. Even when throwing Fire Bombs at these blocks, they still will not go up in flames.
Blocks of Goo - just like their orange and purple variants - can be used as trampolines. If you place them on the ground your player character will not receive any falling damage when landing on them (similar to any kind of liquid). With a little practice you can also use them as a means to jump farther than usually possible, and such you can create "showjumping courses".