Sometimes after using toilet to unrinate and doing istinjaa I can feel drop along my leg or fall below my kness to the calf area. This make me very much confused if it is urine or water from drop after istinjaa. I have tried to sit down for a while after istanja but it happens. This causes a confusion if I am clean or not and that leads me to delaying of prayer. What should I do?

Are stones superior for Istinja استنجاء? >حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ أَخْبَرَنَا مَالِكٌ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا تَوَضَّأَ أَحَدُكُمْ فَلْيَجْعَلْ فِي أَنْفِهِ ثُمَّ لِيَنْثُرْ، وَمَنِ اسْتَجْمَرَ فَلْيُوتِرْ، وَإِذَا اسْتَيْقَظَ أَحَدُكُمْ مِنْ نَوْمِهِ فَلْيَغْسِلْ يَدَهُ قَبْلَ أَنْ يُدْخِلَهَا فِي وَضُوئِهِ، فَإِنَّ أَحَدَكُمْ لاَ يَدْرِي أَيْنَ بَاتَتْ يَدُهُ ‏"‏‏.‏ > > Narrated Abu Huraira: Allah's Messenger (ﷺ) said, "If anyone of you performs ablution he should put water in his nose and then blow it out and whoever cleans his private parts with stones should do so with odd numbers. And whoever wakes up from his sleep should wash his hands before putting them in the water for ablution, because nobody knows where his hands were during sleep." Sahih al-Bukhari 162 https://sunnah.com/bukhari:162

What type of stones are best for Istinjaa استنجاء? Presumably they should be very very *very* slightly rough for traction: https://en.wikipedia.org/wiki/Asperity_(materials_science) ... but of course not too rough. Is there any Sunnah on the particular types, qualities, grades & cuts of the stones? Any records of a particular type and cut of stone being manufactured for istinjaa? Would stones from a riverbed suitable for skipping be ideal? (Germane: I studied metallurgical science in University, so the question is being asked from a Sunnah standpoint, an engineering standpoint, and an end-user standpoint.)