There are no true limits on who can wear black obsidian. Black obsidian is a versatile stone and worn by many. That said, some who practice Feng Shui do have some rules about how black obsidian should be worn. Some believe that anyone over 70 years old should only wear black obsidian on their left hands, and not in the form of a pixiu bracelet. If you don't practice Feng Shui, the only limitation would be your lifestyle. I wouldn't recommend wearing black obsidian during heavy activity or contact sports. Black obsidian is a soft material and also brittle, meaning it could shatter or scratch easily. It's also important to note that, when cleaved, black obsidian has sharp edges. So, in short, only wear black obsidian when it makes sense, and feel free to wear it with whatever you want.