Stones and the disbelievers are the fuel of Hell, as Allah says:

"O you who believe! Save yourselves and your families from a Fire whose fuel is men and stones!" (66:6)

"...then fear the Fire [hell] whose fuel is men and stones, prepared for the disbelievers" (2:24)

What is meant by the people who will be fuel for the Fire is the disbelievers and polytheists. As for the type of stone that will fuel the Fire, Allah (swt) knows best what it really is. Some of the Salaf thought that this stone was brimstone or sulphur. 'Abdullaah ibn Mas'oud said,

"It is a stone of sulphur which Allah (swt) created in the first heaven the day He created the heavens and earth and prepared it for the disbelievers". This was reported by Ibn Jareer, Ibn Abi Haatim and Haakim in al-Mustadrak. Ibn 'Abbaas, Mujaahid and Ibn Juryayj were also of this opinion. (Tafseer Ibn Katheer, 1/107)

If this opinion was based on something that the Prophet (saw) had said then we could accept it without hesitation or argument. If it is the matter of ijtihaad, based on knowledge of the nature and qualities of stones, then we cannot accept it without question. There may be other stones that are superior to sulphur in strength and flammability. The first Muslims saw that sulphur has qualities that do not exist in other stones, so they thought that it would be the fuel of Hell. Ibn Rajab said,

"Most of the mufassireen suggest that what is meant by stones is sulphur, with which the Fire of Hell will be fuelled. It was said that it includes five kinds of torment that are not found in other stones: it catches fire quickly, it has an unpleasant odour, it produces a lot of smoke, it sticks to flesh, and it is very hot when it is heated" (Ibn Rajab, at-Takhweef min an-Naar, p.107).

Allah may create other kinds of stones that are superior to sulphur, and we can be sure that whatever exists in the Hereafter will be different from this world.

Another fuel for the Fire will be the gods, which were worshipped instead of Allah (swt);

"Certainly! You [disbelievers] and that which you are worshipping now besides Allah, are [but] fuel for Hell! [Surely] you will enter it. Had these [idols etc] been aaliha [gods] they would not have entered there [Hell] and all of them will abide therein." (21:98-99)

"al-Jawhaari said, "Everything that is used to feed the Fire or increase it is its fuel". Abu Ubaydah said, "Everything that you throw into the Fire is fuel for it" (Yaqazah uli al-i'tibaar, p.61)