Valued Decor is a mod designed to expand the design and decoration possibilities in Minecraft while preserving the feel of the vanilla game. There are sets of decorative blocks that can be crafted out of non-intrusive worldgen rock types. Various vanilla blocks have been extended on as well for more possibilities.
There are (as of yet) three new rock types that spawn in the world: limestone, slate and quartzite. They all spawn with relatively high frequency and large cluster sizes, but at different levels. The hope is that they don't feel intrusive in your world. They all have decorative blocks ranging from panels to bricks to pillars that can be crafted.
Cement is also added to the game and can be used to make cement blocks and is also a binding agent of sorts for other mod recipes. It has a variety of blocks that can be crafted from it as well.
As of yet, the main vanilla block that is utilized is obsidian; a large range of gilded obsidian are added to the game (For those super-expensive projects)