• قَسَتْ
• قَسْوَة
• قَٰسِيَة

Appears 6 times in the Quran.

1. 2:74 ثُمَّ قَسَتْ قُلُوبُكُم مِّنۢ بَعْدِ ذَٰلِكَ فَهِىَ كَٱلْحِجَارَةِ أَوْ أَشَدُّ قَسْوَةً وَإِنَّ مِنَ ٱلْحِجَارَةِ لَمَا يَتَفَجَّرُ مِنْهُ ٱلْأَنْهَٰرُ وَإِنَّ مِنْهَا لَمَا يَشَّقَّقُ فَيَخْرُجُ مِنْهُ ٱلْمَآءُ وَإِنَّ مِنْهَا لَمَا يَهْبِطُ مِنْ خَشْيَةِ ٱللَّهِ وَمَا ٱللَّهُ بِغَٰفِلٍ عَمَّا تَعْمَلُونَ

   Then your hearts hardened after that, so that they were like rocks, rather worse in hardness; and surely there are some rocks from which streams burst forth, and surely there are some of them which split asunder so water issues out of them, and surely there are some of them which fall down for fear of Allah, and Allah is not at all heedless of what you do.
   

2. 5:13 فَبِمَا نَقْضِهِم مِّيثَٰقَهُمْ لَعَنَّٰهُمْ وَجَعَلْنَا قُلُوبَهُمْ قَٰسِيَةً يُحَرِّفُونَ ٱلْكَلِمَ عَن مَّوَاضِعِهِۦ وَنَسُوا۟ حَظًّا مِّمَّا ذُكِّرُوا۟ بِهِۦ وَلَا تَزَالُ تَطَّلِعُ عَلَىٰ خَآئِنَةٍ مِّنْهُمْ إِلَّا قَلِيلًا مِّنْهُمْ فَٱعْفُ عَنْهُمْ وَٱصْفَحْ إِنَّ ٱللَّهَ يُحِبُّ ٱلْمُحْسِنِينَ

   But on account of their breaking their covenant We cursed them and made their hearts hard; they altered the words from their places and they neglected a portion of what they were reminded of; and you shall always discover treachery in them excepting a few of them; so pardon them and turn away; surely Allah loves those who do good (to others).
   

3. 6:43 فَلَوْلَآ إِذْ جَآءَهُم بَأْسُنَا تَضَرَّعُوا۟ وَلَٰكِن قَسَتْ قُلُوبُهُمْ وَزَيَّنَ لَهُمُ ٱلشَّيْطَٰنُ مَا كَانُوا۟ يَعْمَلُونَ

   Yet why did they not, when Our punishment came to them, humble themselves? But their hearts hardened and the Shaitan made what they did fair-seeming to them.
   

4. 22:53 لِّيَجْعَلَ مَا يُلْقِى ٱلشَّيْطَٰنُ فِتْنَةً لِّلَّذِينَ فِى قُلُوبِهِم مَّرَضٌ وَٱلْقَاسِيَةِ قُلُوبُهُمْ وَإِنَّ ٱلظَّٰلِمِينَ لَفِى شِقَاقٍۭ بَعِيدٍ

   So that He may make what the Shaitan casts a trial for those in whose hearts is disease and those whose hearts are hard; and most surely the unjust are in a great opposition,
   

5. 39:22 أَفَمَن شَرَحَ ٱللَّهُ صَدْرَهُۥ لِلْإِسْلَٰمِ فَهُوَ عَلَىٰ نُورٍ مِّن رَّبِّهِۦ فَوَيْلٌ لِّلْقَٰسِيَةِ قُلُوبُهُم مِّن ذِكْرِ ٱللَّهِ أُو۟لَٰٓئِكَ فِى ضَلَٰلٍ مُّبِينٍ

   What! is he whose heart Allah has opened for Islam so that he is in a light from his Lord (like the hard-hearted)? Nay, woe to those whose hearts are hard against the remembrance of Allah; those are in clear error.
   

6. 57:16 أَلَمْ يَأْنِ لِلَّذِينَ ءَامَنُوٓا۟ أَن تَخْشَعَ قُلُوبُهُمْ لِذِكْرِ ٱللَّهِ وَمَا نَزَلَ مِنَ ٱلْحَقِّ وَلَا يَكُونُوا۟ كَٱلَّذِينَ أُوتُوا۟ ٱلْكِتَٰبَ مِن قَبْلُ فَطَالَ عَلَيْهِمُ ٱلْأَمَدُ فَقَسَتْ قُلُوبُهُمْ وَكَثِيرٌ مِّنْهُمْ فَٰسِقُونَ

   Has not the time yet come for those who believe that their hearts should be humble for the remembrance of Allah and what has come down of the truth? And that they should not be like those who were given the Book before, but the time became prolonged to them, so their hearts hardened, and most of them are transgressors.