Which is the best weapon against common invaders / unwanted visitors in dwarf fortress

Ok, I'm going to give you a crash course in Geology.

In traditional Geology there are three kinds of Rock: Sedimentary, Metamorphic and Igneous (actually that's an over simplification, but pretend its true). Dwarf Fortress is modeled on this system but it further breaks Igneous down into Igneous Extrusive and Igneous Intrusive (Lava that cooled on the surface and lava that cooled underground, obviously magma is found in the latter). To these layers DF also adds Soil and Aquifers. So our full list is:

• Sedimentary
• Metamorphic
• Igneous Extrusive
• Igneous Intrusive
• Soil
• Aquifer

Now on your embark screen you'll not see these layers, instead you'll see things like: Gabbro, Gneiss, Shale, Basalt, etc. Each of these indicates one of the above layers,

• Shale -> Sedimentary
• Basalt -> Extrusive
• Gabbro -> Intrusive
• Gneiss -> Metamorphic

So why describe them as certain stones instead of what the layer is? Well, the stone indicated is the most common stone in that layer, it is NOT the only stone in that layer. In addition to deposits (Jet, Gold, Hematite, etc) any other type of similar stone can be found.

So how do you tell if you have Marble? Well if you see Marble on your embark screen, that's a big hint (but you said you didn't). So the next thing you need to look for is a similar stone. Marble happens to be a Metamorphic rock, so it can be found if you have other Metamorphic rock, namely: Quartzite, Slate, Phyllite, Schist, and Gneiss. If you have none of those, then the answer is NO. If you have at least one then the answer is MAYBE.

Only way to tell in a MAYBE circumstance is to dig... Sorry

For those of you who actually care more about Rock, I've defined the terms further here. All rock essentially starts at as Magma. As Magma is heated by the pressure at the Earth's core it rises (remember 10th Grade Chemistry? Hot things rise!). It eventually reaches the surface and becomes Igneous Rock. Whether it becomes Intrusive or Extrusive has to do with whether it makes it all the way to the top. So we can say, all rock starts as Magma and becomes Igneous.

From there it can be weathered down by the elements, earth, rain, water, etc. This weathered down rock becomes sand or soil. These soils build up over time and eventually are submerged below other soil (or sometimes other rock). Eventually enough pressure builds up and the soil becomes hard and becomes a rock known as Sandstone. Sandstone is soil that has been compressed back into rock.

In a world without continents that's all there would be, but in our Earth we have massive continents. These continents are formed of large rock beds deep within the Earth's crust. As they heat they rise to the surface. At the surface they lose heat and cool. Eventually they cool so much they are subducted by newer Continents. This is not a subtle process, but actually a very violent one, with massive forces of pressure, stress and strain. These forces warp Igneous and Sandstone compacting it. The end result is a new kind of stone called Metamorphic rock (meta is Greek for change, and morph for form). Its usually identifiable by the stress lines present.

Congrats, that was your crash course in Geology, here is your Diploma.

The Advanced Class!

So someone asked about Flux and I figured given its importance in DF it might be worthwhile for me to explain what Flux is. Flux is often an ambiguous term give its preference in different scientific disciplines. The Flux referred to by DF is the term from Metallurgy. In metallurgy flux is a substance which removes oxidation from metals, usually used in soldering, brazing, and welding. Additionally, flux is used in smelting to remove impurities from the metal (also to liquefy slag, but that a different discussion). In modern steel making we use burnt lime or dolomite as a flux material.

Even though steel making is an industrial technology, the Dwarves natural instincts with relationship to rock have given them an innate understanding of the necessary complexities of steel making. As a result, Dwarven steel is far more pure than the period would otherwise suggest (modern steel making wasn't invented till the 17th century).

In the newer versions (specifically 0.31.16 which I'm currently playing), you can specify an individual Dwarf's equipment by type, or even down to the specific item. From the (current at the time of writing) Dwarf Fortress wiki:

In the military screen, press n to open the uniforms tab. Each listing under the 'Uniforms' header is essentially a pre-designed set of equipment that you can quickly apply to any individual soldier or entire squad, much like a template. You can create new uniforms if you want and add or remove items from any uniform set by navigating this menu. Uniform templates are only created in this tab, not applied.

While still in the military screen, press e to open the equipment tab. The default sub-tab, 'View/Customize will be open. In this screen you can select individual dwarves and apply individual pieces of equipment to them, from Armor, Leggings, Helms, Gloves, Boots, SShields, and Weapons, as well as Material and Color depending on the piece of equipment highlighted. To select a specific piece of equipment (such as an artifact), select 'specific _____' under that equipment type (eg. 'specific armor' in the Armor field); for your convenience, highest-quality equipment is listed first.