IIRC Volcanic has exactly twice the modulus and twice the strain@yield [relative to steel]; the way Toady defines his math, the two factors of "2" cancel out. So if you apply the same amount of stress that would cause the steel to fail, the volcanic will have deformed exactly the same amount (but not fail.)
What? 
Toady uses a material model based on a stress (force per area, or "Pascals") vs. strain (change in length per original length) curve,
http://www.mathalino.com/reviewer/mechanics-and-strength-of-materials/stress-strain-diagramHe treats it as if there is a linear relationship between the stress and strain up until the "Yield point". That's called the elastic region in real materials -- the material will deform like a spring proportional to the amount of force per area you apply. If you apply even more force beyond the Yield poitn the material will deform more and more, what is called Plastic deformation, until it reaches the ultimate failure or fracture point.
In the raws, Toady defines the Yield point and the Fracture point both in some unit that measures stress, probably kiloPascals. That's pretty straightforward, bigger numbers are better, indicating the material is stronger. But the slope of the linear "elastic" part of the curve matters too -- if you have a material that is tough but "gives way" a lot, you might not get sliced by the sword, but your armor will deform and you'll get badly bruised or broken. DF combat models this too.
In engineering handbooks you would usually find the elastic modulus -- i.e. the slope of the line, which is the stress divided by the strain at the Yield point -- which is a good metric for the stiffness of a material. Toady decided rather than putting the modulus in raws instead to use "strain at yield", which is sort of equivalent, but you have to divide this number into the Yield point to find the modulus. Therefore, like in Volcanic when you doubled both numbers, the result is twice the strength, but exactly the same stiffness because the two factors of 2 cancel each other.
As I've mentioned before this system gives some room for materials that differ from regular metals in interesting ways. I.e. you could have materials that are pretty tough but not stiff -- like organic materials in the real world. Or you could have materials that are very strong up to their yield point but then fail catastrophically with little or no Plastic deformation -- like glass in the real world. This is how I would design my "Elder Scrolls" inspired materials to be similar "tier" in strength but also different than regular metals (bonemold or ironbone, ashglass, ebonglass). I don't know for sure how noticeable these distinctions would be in the DF engine... at the end of the day the strength is probably far and away the main thing -- but who knows?
Since you doubled the good and doubled the bad, your end result is basically steel.
They're equal only in stiffness; volcanic is still twice as good in strength. It's still much better.
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Topic: Future of the mod - Progress & Suggestions - race poll (Read 74496 times)