What is meant by the following aya from
'Anzala Mina As-Samā'i Mā'an Fasālat 'Awdiyatun Biqadarihā Fāĥtamala As-Saylu Zabadāan Rābīāan ۚ Wa Mimmā Yūqidūna `Alayhi Fī An-Nāri Abtighā'a Ĥilyatin 'Aw Matā`in Zabadun Mithluhu ۚ Kadhālika Yađribu Allāhu Al-Ĥaqqa Wa Al-Bāţila ۚFa'ammā Az-Zabadu Fayadh habu Jufā'an ۖWa 'Ammā Mā Yanfa`u An-Nāsa Fayamkuthu Fī Al-'Arđi ۚ Kadhālika Yađribu Allāhu Al-'Amthāla
He sends down water from the sky, and riverbeds flow according to their capacity. The current carries swelling froth. And from what they heat in fire of ornaments or utensils comes a similar froth. Thus God exemplifies truth and falsehood. As for the froth, it is swept away, but what benefits the people remains in the ground. Thus God presents the analogies. 013-017
أَنزَلَ مِنَ السَّمَاءِ مَاء ً فَسَالَتْ أَوْدِيَةٌ بِقَدَرِهَا فَاحْتَمَلَ السَّيْلُ زَبَدا ً رَابِيا ً ۚوَمِمَّا يُوقِدُونَ عَلَيْهِ فِي النَّارِ ابْتِغَاءَ حِلْيَةٍ أَوْ مَتَاعٍ زَبَدٌ مِثْلُهُ ۚ كَذَلِكَ يَضْرِبُاللَّهُ الْحَقَّ وَالْبَاطِلَ ۚ فَأَمَّا الزَّبَدُ فَيَذْهَبُ جُفَاء ً ۖ وَأَمَّا مَا يَنفَعُ النَّاسَ فَيَمْكُثُ فِي الأَرْضِ ۚ كَذَلِكَ يَضْرِبُاللَّهُ الأَمْثَالَ
What is this "similar froth"; "zabadun"; that is being referred to? The first half of the verse I sort of understood to mean the slag oxides that may occur when smelting metal from ore, but then in second to last sentence it says "the froth is swept away, but what benefits people remains in the ground", possibly eluding to the process of sedimentary mineral formation.
I realize that there is a much more important spiritual meaning of this aya. Information on the events and context of this aya's revelation would be deeply appreciated.
