More tests with spears confirm something nice: For the most part, whether or not a material can penetrate armor of a different material actually makes sense.
Basically: Tin and gold armor are trash, steel is better than bronze which is better than iron, platinum is sort of on par with iron but can't penetrate it and is absurdly heavy and expensive to use, etc.
Also, I could swear that tin manages to block gold but absorb barely any of the shock, which is interesting. The test dwarves are breaking bones with their gold spears through tin chain, while being totally unable to penetrate. So it seems like different materials absorb shock quite differently in the event of non-penetrating hits.
Adamantine spears penetrate like crazy, tearing up the insides of people's chests straight through steel breastplates, and going right through chain leggings to shatter bones and rip tendons.
Adamantine armor stops even adamantine from penetrating, but golden spears can still manage to break bones through sheer force, through adamantine chain leggings, despite being totally worthless when it comes to penetration. This only seems to happen extremely rarely, though, so it's not a big deal. It's that old "who cares if Frodo's shirt is mithril, that spear should have caved in his chest!" principle at work, I guess, but most of the time the adamantine just plain deflects gold and even platinum.
So the "what can penetrate what?" table, in itself, doesn't seem bad, except that it seems too much like you either can or can't penetrate a given material with a given other material, without much wiggle room where you do rarely, or only sometimes.
[EDIT, AGAIN, I'M BAD AT THIS]
~FURTHER CONCLUSIONS~
It seems that density affects how much force gets transferred through layers. Regular tin chain stops gold spears from penetrating, but the gold still transfers enough force to break bones sometimes. With tin modified to have ten times the density (yowza!), the gold can't do much more than bruise fat through it. Also, this SEEMS to make it slightly more difficult for weapons to penetrate effectively as well, but by how much, I'm not sure.
These conclusions are realistic enough, so I'm pretty happy.
Tin in general seems to still deflect iron sometimes, but that's fine. It doesn't most of the time.
One problem I've noticed, if it's indeed a problem, is that aluminum weapons seem almost entirely useless. A sharpened aluminum spear should be able to significantly injure someone, but it can't. Same goes for tin, really. Maybe density counts for too much, here? Also, aluminum armor seems less useful than gold armor, which... SEEMS unrealistic, but it's hard for me to tell.
[I DON'T FEEL LIKE DOUBLE-POSTING]
Okay, it seems like dwarven skin might be too weak in general? Aluminum and tine aside (which are strange enough), cheetah claws only dent the skin (no bleeding or bruising at all), and GCS fangs literally glance off every single time.
Conversely, chitin seems too weak. A dwarven fist can easily tear through the fat underneath it. A punching dwarf in arena mode is very easily tearing straight through GCS and antman body parts, despite being unable to do anything but bruise other creatures.
FWIW, on the subject of cumulative damage, bruises getting worse with successive hits should result in some sort of serious internal bleeding and function loss, particularly when it's muscle, not to mention compound organ/brain impairment with each successive strike. I have no idea how much of this exists in the current system, but it would be a way for repeated blunt trauma (e.g. fistfights) to actually matter.