From reading the wiki, it seems that iron ores can be found in layers consisting of chalk. I've now mined out almost two complete layers of chalk, and found nothing but gems. Is it possible that my map doesn't have any iron ore, or am I misunderstanding how to find the ore?
The first couple of layers consist of chalk, so that's where I'm digging. Unfortunately the strip mining makes the fortress quite ugly, as that's where I've got my workshops. The lower levels have mainly phyllite and schist.
Best Answer
Finding precious ores and stones doesn't have to be too messy, various techniques have been developed that balance chances of discovering minerals and beauty, here's the simplest one:
A 7x7 mining pattern will reveal all veins and some clusters, leaving space for 5x5 rooms to be mined later. The width of unmined squares can be changed according to your needs.
You can use DFHack's prospect utility to see the type and number of minable raw materials. You can also use the reveal utility to see exactly where everything is, though doing so may result in bugs. DFHack is compatible with the current version of DF (0.34.07).