This has come up again and again from start. Take a rock the same size as a sledge hammers head, and compare them. average hard rock (ores can be a bit more in wait) has about 3,2 kg to 3,6 kg per dm³ Iron has 7.3 kg per dm³. So basicly if you whant realism what @Lefee-(SWE) sayed, be happy it´s not half a copper that dropps out of you´re furnace.