Mastery does basically nothing for normal rock (on homeworlds anyway. Other world rocks may have “hardness” now, or just more HP. I haven’t really tried them yet,), but in the case of ores, they seem to have a “hardness” quality in addition to straight HP.
Say that a block has, for example, 2,000 HP and 0 hardness. Your hammer power times your damage bonus from the Power attribute will be directly applied to that. The equation is probably something like 500 hammer power times Attribute Power(let’s say +50%) bonus divided by (hardness + 1). With zero hardness, this is equivalent to dividing by one. Each strike will do 750 damage, because that’s 150% of 500.
On the other hand, say that the block has 2,000 HP and 1 Hardness. Your damage is (500 x 1.5) = 750. With no Hammer Mastery, you get no mitigation of that Hardness. Your damage is divided by (1 + 1 hardness) so you do half damage, or 375 damage per hit.
I’m fairly certain that if this is similar to the actual equation, “Hardness” does not go above 3. I’d need to do a lot more testing to figure out the exact numbers and equation though.
I’d also suspect that, like slingbows, Mastery only has an effect on low-quality hammers in low quantities. Say, a Wooden hammer can only use 1-2 ranks of mastery no matter how many you have, whereas a Titanium hammer can use a max of perhaps 5-6 ranks, and a Gem hammer is in the 8-10 range.
Thus, higher ranks of Hammer Mastery will do you no good if you’re not using better hammers.
Edit: The equation probably subtracts something like 0.33 or 0.25 from the “Hardness” of a block per rank of your Mastery that your current hammer is able to use.
The “Hardness” part of the equation may also be something a bit more nuanced, such as (1+ [0.5 x Hardness]), in which case “Hardness” values probably go up to six or seven-ish. It could also be something much simpler like a flat damage reduction that is reduced by Mastery on applicable hammer qualities.