Flux
From DwarfFortressWiki
Stones which are classified as flux materials can be used in the creation of pig iron and steel at a smelter. They are considered economic stones. Unlike common rock they all have a material value of 2 instead of 1.
The following materials fit in this reaction class:
Flux is usually found in abundance if present and thus is a preferable building and crafting material over any other stone that has a material value of only 1; rooms, crafts and furniture made from it will therefore have twice the value.
Most flux-types form sedimentary layers (limestone, dolomite and chalk) or are present as inclusions in them (calcite) and thus are seen in the embark-screen. Marble is a metamorphosed sedimentary rock so it can be found from almost any biome. It can be found near igneous extrusive layers i.e. near volcanoes and magma vents.
[edit] Trading for flux
If your map has no flux on it (which is a common situation), then you can request it from the dwarven traders (assuming the traders have access to flux).
