Or HaChaim on Exodus 19:18/אור החיים על שמות י״ט:י״ח

1 והר סיני עשן כלו, and Mount Sinai was completely wrapped in smoke, etc. Fire dominated the Mountain itself and burned its stones so that they turned into limestone, just as in a furnace where stones are burned to make lime. The stones of Sinai became lime. When the Torah says: "the whole Mountain trembled," this is a description of the reaction of the stones when fire dominates them just as the stones in a furnace. The sounds that the stones give off are similar to the sound of someone trembling.