Emerald Block (NOT the vanilla block) is one of the many Decoration Blocks added by the Soul Forest Dimension. It can also be called a Storage Block.
Aswell as most of the Decoration Blocks this block is crafted with 9x Emerald Ingots. After an Emerald Block is made, its content (the ingots) can be easily re-obtained by simply placing the block in a crafting table.
The only use (for now) is the that you can easily store a lot of Emerald Ingots by turning them into blocks.