Grass Block.

Grass appears to be a dirt block with a blanket of a grass-like texture over it. Dirt Blocks on non-superflat modes when in direct sunlight will become grass blocks over time.

This block cannot be mined, however, because it will become a normal dirt block. It can only be obtained from spiders without having to grow them.

They can only be bought in the Block Shop in Creative Mode only for 30c.

Saplings and flowers can only be placed on a grass block or a dirt block. Saplings, once they have became trees, will replace the grass block they were placed on with a block of wood