Is the flint used in disposable lighters toxic?
The "flint" in disposable lighters is made of pyrophoric alloy called "ferrocerium", which consists of approx. 50% cerium, 25% lanthanum and 20% iron; the remaining 5% is magnesium, neodymium and praseodymium. Exact composition may vary depending on the manufacturer.

If you rub such an alloy on a rough surface, tiny metal particles are formed, which are instantly ignited and produce metal oxides. Neither of the expected products are particularly toxic: lanthanoid oxides, particularly $\ce{CeO2}$ and $\ce{La2O3}$, are not toxic and can only cause mild irritation when swallowed, and iron oxide $\ce{Fe3O4}$ is basically rust.

To sum it up, unless you are allergic to transition metals (which people usually discover very early), I cannot think of any health hazards coming from the "flint" or the sparks.