مِّن سِجِّيْلٍ

مِّن سِجِّيْلٍ is a phrase (شِبْهُ الجُمْلَةِ), a compound consisting of a particle of pulling down (preposition) and a genitive noun

سِجِّيْلٌ is the natural state of this indefinite noun and it changes to مِّن سِجِّيْلٍ due to the preposition that precedes it and مِن takes a tanween (مِّن) due to the indefinite genitive noun preceding it

PhraseParticle/NounNoun Particle 
شِبْهُ الجُمْلَةِجَارٌ وَمَجْرُوْرٌاسْمٌ مَجْرُوْرٌحَرْفُ جَرٍّ
PhraseCompoundGenitive NounParticle of pulling down
  
from clayمِّن سِجِّيْلٍ=سِجِّيْلٍ+مِّن
from clayclayfrom
Surah/Below is the only verse in the holy Quran containing the genitive particle and noun compoundمِّن سِجِّيْلٍ
Verse
105-4Throwing on them stones of clayتَرْمِيْهِم بِحِجَارَةٍ مِّنْ سِجِّيْلٍ