Over time, carbon-14 decays radioactively and turns into nitrogen. Because of the short length of the carbon-14 half-life, carbon dating is only accurate for items that are thousands to tens of thousands of years old. Most rocks of interest are much older than this.

Is carbon-14 suitable for dating most rocks?

Carbon-14 has a relatively short half-life. It decays quickly compared to some other unstable isotopes. So carbon-14 dating is useful for specimens younger than 50,000 years old. Thats a blink of an eye in geologic time.