Unfortunately there is no fire stone in giant chasm....I looked inside and outside but no reason...
You can go to the Scientist assistant and answer his question with Pansear and he'll give you a Fire Stone. You can go to certain caves with dust clouds in them, such as Wellspring Cave, Chargestone Cave, Mistralton Cave, Twist Mountain, Challenger's Cave, Victory Road, and Giant Chasm. You can also find it in Desert Resort.
In Pokémon Black and White, you can find Fire Stones in Dust clouds that will materialize in Twist Mountain, Victory Road, Giant Chasm, Challenger's Cave, Wellspring Cave, Chargestone Cave and Mistralton Cave. Additionally you can find one in the Desert Resort and you can find an extra one from the guy in Castelia City in the event of answering Pansear to his question.
go underground and keep digging till u find a water stone or a fire stone
you find them underground
You can find a fire stone at Fuego Ironworks, Stark Mountain (With Dowsing Machine).
in white two, you can find it in the desert resort
In Hiun City / Castelia City (japanese/english version), there is a pier near the gate to the Sky Arrow Bridge. Talk to the scientist there, and answer Pansear to get a Fire Stone. You can also get one in the Desert Resort. There are also "invisible" Fire Stones in these places: -Wellspring Cave -Chargestone Cave -Mistralton Cave -Twist Mountain -Challenger's Cave -Victory Road -Giant Chasm
down in the fire
You can buy a Fire Stone in the Pokeathlon Athlete Shop on Tuesdays for 2500 points.
You can find then in the underground.
iron works
in pokemon silver how to get a fire stone is where the weired tree a kid name alan will give you a fire stone if you give him your number