A response team for nuclear, radiological, biological and chemical emergencies was called, one floor of the Lyon court house evacuated and about 50 people checked for radiation exposure, all negative.
Claiming to promote world peace, the man smashed a display case with a glass front, releasing pigments of autunite, a greenish fluorescent mineral, Lyon firefighter Colonel Bertrand Kaiser told AFP.
This "caused a minor panic," he said, adding that one floor of the courthouse was evacuated.
The man, who had been involved in a commercial court case, was arrested.
The material was taken for analysis by the Nuclear Safety Authority.