Recently, while hard soldering with a butane torch, I noticed an old lighter flint on a firebrick and began to play the torch on it. The flint became red hot, but didn't melt. When I removed the flame, however, the temperature of the flint increased until it glowed bright orange for several seconds before cooling to dull red. Then, to my surprise, it slowly began to get hotter again until it glowed bright red. It increased in size and changed in texture to resemble crumbly black coke.
Lighter flints are an alloy of iron and a lanthanide element, mainly cerium.
These are extremely reactive metals and it is the exothermic oxidation of small, friction-heated particles ejected by the lighter's grinding wheel which produces sparks. The butane gas flame will exclude oxygen as it heats the flint but when removed oxidation and heat evolution follow rapidly. An oxide layer builds up and slows the ingress of oxygen, allowing some cooling, but the oxide coating then cracks and reaction and heating start again until the metals are consumed and only the mixed oxide cinder remains. Its pyrophoric property was discovered in 1903 by Auer von Welsbach. Cerium alone is too soft, so iron is used to harden it.