The laws of science are variable in D&D settings. Gunpowder, for example, doesn't function on Faerun. You need smokepowder, a magical substance that fills the same role (although I've read that jeweler's rouge will also work).
Diamonds might be compressed carbon. They could alternately be the crystallized essence of slain angels.
I think it's safer to say that real world science applies so long as the DM hasn't decided otherwise, regardless of whether or not the players are aware. It might be reasonable for a player to assume that their characters don't fly off into the void due to gravity, but it could be equally true that gravity doesn't exist in the setting and that the reason they don't fly off is that the world is flat, or because they live on the tip of a rocket that is in a state of constant acceleration.
Heck, given magic, you'd be hard pressed to come up with an explanation for why the various laws of science aren't merely strongly worded suggestions in a D&D universe. There are plenty of spells and other magic that seemingly contravene science as we understand it. Gargantuan dragons capable of flight, for example.