If you notice a black ring developing under the linoleum around the base of your toilet, you probably need to replace the wax ring. The ring is supposed to seal the toilet to the floor. If it starts to leak, sewage, mold, or bacteria can develop under the flooring and cause a black ring. To replace the wax ring, you will need to pull the toilet off the floor so that you can install the new one. Since you are removing the toilet from the floor anyway, it is a good idea to ensure that the drain line underneath the toilet is clear. If there is an obstruction in the drain line, the toilet will not flush correctly. This can allow pressure to build up, which can cause the wax ring to leak. If you are not experienced in working with home plumbing, it is usually best to contact a professional to handle the repair.