Can wearing the wrong gemstone cause problems?

Classical texts caution that it can. Strengthening a planet that is already a functional malefic for a given Ascendant, or that is severely afflicted, is generally considered more likely to amplify that planet's difficult themes rather than its beneficial ones. This is the main reason gemstone therapy is treated as a targeted remedy based on a full chart reading, rather than a one-size-fits-all accessory.