Violet 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 Violet Gems. After a Violet 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 Violet gems by turning them into blocks.