Are there precautions to take before wearing a gemstone?

Yes. Classical practice recommends testing a stone briefly before committing to it, since a gem meant for one chart can feel physically or emotionally uncomfortable if it doesn't suit the wearer. Astrologers also pay attention to stone quality (natural versus treated), correct weight relative to the person, the metal it's set in, and the specific day, time, and finger or method of wearing it — all of which are said to affect how effectively the stone channels its planetary influence.