Allaah (The Most High) stated about those who disobeyed Prophet Musaa (alayhis-salaam):
ثُمَّ قَسَتْ قُلُوبُكُم مِّن بَعْدِ ذَٰلِكَ فَهِيَ كَالْحِجَارَةِ أَوْ أَشَدُّ قَسْوَةً
Then, after that, your hearts were hardened and became as stones or even worse in hardness. [2:74]
Imaam Ibnul Qayyim (rahimahullaa) stated: The hardness of their hearts does not become lesser than the hardness of stones, rather if it does not become harder than stones, it does not become lesser than it.
Source: Badaai Tafseer Al-Jaami Limaa Fassarahu al-Imaam Ibnul Qayyim’ Vol 1, page 130; abridged and slightly paraphrased