Limestone spawned as part of worldcreation has a flat beige color.
Once mined and placed, it is crossed by lines that make it resemble shaped bricks.
Where Found
Limestone is found underground, almost always in sharply or slowly curving veins of varying width that may parallel or intersect caves. It is often accompanied by other "soft" stone blocks such as marble, red marble, oil, lapis lazuli, or sandstone.
It can be found at any depth, although it may be more common near the surface.
It is easiest to find in a "already formed" mine. Look for a mine that you didn't make, then go in it and you will most likely find it quickly.
Uses
Like most blocks, limestone can be used in construction.