Chalk is a flux stone found in the sedimentary layer. As such, it is worth twice as much as most other stones.
Despite chalk's rather chalky nature in real life, furniture and crafts made out of chalk in Dwarf Fortress last as long as granite ones.
Chalk is not magma-safe material.
 In Real Life
Chalk, a soft white sedimentary rock, is a form of limestone composed of the mineral calcite. It often contains chert and minor amounts of silt and clay.
Chalk blocks (white) in a wall