Lord Kamina
Not an engineer.
Bonus Stacking
Bonuses from equipment (including magical material bonuses) do not stack with one another; the character enjoys only the best among his various modifiers. For example, a Dragon- Blooded character with a jade daiklave and jade hearthstone bracers would only lower his Speed by one point—the two magical material bonuses will not stack to produce Speed -2. Likewise, a character wearing lamellar armor and a magic girdle that provides armored soak enjoys only the higher of the two soak ratings—they do not stack together.
I'd say that hearthstones definitely count as magical equipment, no?
Bonuses from equipment (including magical material bonuses) do not stack with one another; the character enjoys only the best among his various modifiers. For example, a Dragon- Blooded character with a jade daiklave and jade hearthstone bracers would only lower his Speed by one point—the two magical material bonuses will not stack to produce Speed -2. Likewise, a character wearing lamellar armor and a magic girdle that provides armored soak enjoys only the higher of the two soak ratings—they do not stack together.
I'd say that hearthstones definitely count as magical equipment, no?