There is no separate texture for it, the texture it uses is Stone.
And, in case you or anyone else was wondering the following objects don't have a separate texture either:
Fence = Wooden Planks
Wooden Pressure Plate = Wooden Planks
Stone Pressure Plate = Stone
Sandstone Slab = Sandstone
Wooden Slab = Wooden Planks
Cobblestone Slab = Cobblestone
Bookshelf (top) = Wooden Planks
Lever (box) = Cobblestone
Wooden Stairs = Wooden Planks
Cobblestone Stairs = Cobblestone
Thanks in advance!
And, in case you or anyone else was wondering the following objects don't have a separate texture either:
Fence = Wooden Planks
Wooden Pressure Plate = Wooden Planks
Stone Pressure Plate = Stone
Sandstone Slab = Sandstone
Wooden Slab = Wooden Planks
Cobblestone Slab = Cobblestone
Bookshelf (top) = Wooden Planks
Lever (box) = Cobblestone
Wooden Stairs = Wooden Planks
Cobblestone Stairs = Cobblestone
Can't think of any others.