ثُمَّ قَسَتْ قُلُوبُكُم مِّن بَعْدِ ذَٰلِكَ فَهِيَ كَالْحِجَارَةِ أَوْ أَشَدُّ قَسْوَةً ۚ وَإِنَّ مِنَ الْحِجَارَةِ لَمَا يَتَفَجَّرُ مِنْهُ الْأَنْهَارُ ۚ وَإِنَّ مِنْهَا لَمَا يَشَّقَّقُ فَيَخْرُجُ مِنْهُ الْمَاءُ ۚ وَإِنَّ مِنْهَا لَمَا يَهْبِطُ مِنْ خَشْيَةِ اللَّهِ ۗ وَمَا اللَّهُ بِغَافِلٍ عَمَّا تَعْمَلُونَ
Summa qasat quloobukum mim ba'di zaalika fahiya kalhijaarati aw ashaddu qaswah; wa inna minal hijaarati lamaa yatafajjaru minhul anhaar; wa inna minhaa lamaa yashshaqqaqu fa yakhruju minhul maaa'u wa inna minhaa lamaa yahbitu min khashyatil laah; wa mallaahu bighaafilin 'ammaa ta'maloon
Then, even after all that, your hearts hardened — becoming like stones, or harder still. For there are stones from which rivers burst forth, and others that split open so water flows from them, and others that fall down in awe of God. And God is not unaware of what you do.
The comparison grants stones a responsiveness it withholds from hearts, and it does so in three stages: some stones release rivers, some split so water works its way out, and some fall from awe. Each stage is a lesser motion than the one before, so even the slightest yielding is set above what is being described. The closing clause moves from the image to the knowledge that observes it.