Rocks with high concentrations of K₂O, Na₂O, SiO₂ and Al₂O₃ will generally contain low density minerals like quartz and alkali (Na and K), rich feldspar.

Rocks with high concentrations of FeO, Fe₂O₃, TiO₂ and CaO will posses a mineralogical makeup that makes them dense having minerals like olivine, pyroxene and magnetite.

MgO is neutral as far as density goes although Mg TENDS to be correlated with FeO and CaO.

Overall, if a rock has heavier elements like lead, iron or magnesium, it will have a higher density. If you pick up a rock with these minerals, it will feel very heavy to hold. Rocks that don't have these heavy elements instead have lighter elements like silicon, oxygen, potassium and aluminum, so they are less dense (and will feel lighter when you pick them up).