Olivine 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 Olivine Gems. After an Olivine Block is made, its content (the gems) 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 Olivine Gems by turning them into blocks.