Something that's made of or contains flint, or hard silica rock, is flinty. Flinty soil is rough and rocky, and not the best for planting a garden.
Flinty earth is studded with jagged flakes of flint, and the adjective is often used in a figurative way to capture an equally rough and stony mood or personality. Your flinty uncle, for example, might be gruff and unsmiling. Since the 16th century flinty has been used this way, to mean "hard-hearted" or "tough as flint." The "full of flint" meaning is slightly newer.