If you place a normal quartz block or a double quartz slab, it has the same exact textures on all sides, except the bottom. The bottom texture will be referred to as 'Texture 0', and the other one as 'Texture 1'

The Smooth Quartz currently has 'Texture 1' on all of it's sides, even the bottom.

Here comes the bug, I think it's supposed to have 'Texture 0' on all sides.

And if we are here, a sidenote, that it would be cool to see a block with 'Texture 1' on all of it's sides as well, but this is not part of the bug report itself.