A few people get a green mark on their skin when they wear copper. This is in no way considered harmful, in fact many consider the copper to be beneficial for certain conditions. My mother had arthritis in her hands and felt that wearing a copper bangle all the time was very helpful. The mark will easily wash off. If wearing a bracelet that may leave a mark on your skin would bother you, please select a silver, gold or hematite bracelet instead.
