I know this is a little bit late and possibly flogging a skeletal horse... but something for those with a stronger science background than myself: Doesn't adamantine's molar mass (same as that of iron) rule out possible exotic properties that were brought up to justify its edge value? If I'm talking nonsense, corrections are very welcome.
Obsidian is 13 times as dense as adamantine, with comparable molar mass (actually lower in-game). So obsidian's smallest 'elementary entities' (mostly silicon dioxide molecules) should be packed more tightly than those of adamantine (whatever their nature).
Given that a monomolecular edge is possible for obsidian blades (and something fairly close is achievable through old-fashioned knapping, if the value takes practicality into account as well as theoretical limits) I can't see a way to justify 20000 for obsidian and 100000 for adamantine.
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Anyway, harping about adamantine aside... has anyone found a good resource for overall material values yet? The system isn't being used to its fullest atm. For example, in many cases no real distinction is made between different strenghts: impact strength set to 3.5 times tensile strength (most readily available figure), most others set equal to tensile strength. I'm also curious about obsidian as a weapon material using its true properties (which I haven't found anywhere) rather than taking everything from marble.