1. It can be assumed that what A said is true, so either B took the jewelry or C took it. Since both B and C told lies, B said that A took it was a lie, and C said that it was a lie, so C took the jewelry.

2. Assuming that B is telling the truth, according to what B said, it can be proved that A is lying, and C said that I didn't take it seriously, so the conditions are not met, and this situation can be ruled out.

Suppose what C said is true, and what A and B said is a lie. Since C didn't take it, it must be A and B, because both of them are lies. The contradiction between A and B is that A says I took it and B says it's true. Because only the combination is true and the condition is not negative, it is excluded.

To sum up, it can be said that A took it.