Copper, in it's varied forms, is possibly
the most common of all ores, all things equal. (That is, without choosing a specific mix of layers.) So you need to pay more attention to those layers at embark, and start to understand what they imply for the future of your exploratory mining and your fortress.
If I want a good variety of ores (and I usually do), I hope for several types of layers when I embark for an "optimal" mix :
- 1) Sedimentary layer(s) - for magnetite pockets (= massive iron, and possibly platinum), bauxite, coals, and often tetrahedrite (which can mean some silver). Unless it's also flux, most all sedimentary is same same.
If I'm lucky enough to get a sedimentary that is
also flux (dolomite, chalk, and/or limestone) that's two birds with one stone - as it were. One of those 3 plus another sedimentary layer is bonus land.
- 2) Flux layer(s) - to make steel from all the iron, natch. This is a dealbreaker for me, but not everyone bashes steel, admittedly. Marble is the best - it's both flux, sedimentary and metamorphic - which makes it both rich and varied. Any flux is a value 2 stone, doubling the value of all rooms, engravings, workshops, constructions, and stone furniture and crafts - created wealth starts to jump.
- 3) Granite layer(s) - tends to be a uniquely rich layer, relative to others
- 4) Gabbro layer(s) - another rich layer, and the exclusive source of some gems/ores.
(Both 3 & 4 are "igneous intrusive" layers; diorite is as well, but, to use the technical geological term, diorite sucks, as compared to the other two.)
If/when I find this mix, I'm confident that I'm set. Any others are then bonus or ignored/tolerated (and you work on your exploratory mining techniques to locate the layers you really want.)
I also prefer to see a lot of height variation, so there are more layers exposed on the surface - makes it easy to find/mine stuff early on. (I like to dig tunnels to exposed veins and work them from the inside - more exploration, and safety for miners and haulers. I can also put a door on that tunnel and lock miners inside to get the job done, or keep experienced miners on the ore while trainees are, well, training, elsewhere.)
If you're having trouble with all the different layers and stones, bookmark the "non-dwarf's guide to stone" in the wiki (there's a link on the main page, under "Guides", top center). If you see a new stone, a layer, ore or pocket, it will tell you in simple terms whether you should care, and why.
(edit - misleading typo, fixed)