Even if calcium deposits have taken up residence in your toilet bowl, there's no need to throw your hands up in the air and purchase a new one. Instead, employ one or more of these techniques, and you're sure to see some squeaky-clean results. Well, for a toilet, anyway.
First, try the green route! A mild, natural acid, such as lemon juice or vinegar, can be very effective at nipping calcium deposits in the bud. Depending on the severity of the stain, the liquid might have to be reapplied and scrubbed vigorously several times with sandpaper or a rag.
Another option that many DIY experts swear by is plumber's cloth. Available at any hardware store, you can just use a section with plain old water to make stubborn stains disappear.
If you don't mind a few common household chemicals, pick up a bottle of commercial toilet bowl cleaner for the task. Some users prefer to drain the toilet bowl first because it prevents the water from diluting the power of the cleaner. Tank drop-ins are also effective at cleaning and preventing stains, although it might take them some time to completely erase the blemishes from your bowl.
If none of the easier options are doing the trick, there are a couple of riskier ways to get the results you desire. For example, muriatic acid can get rid of calcium deposits in mere seconds, but it's also very hard on porcelain and dangerous to use if you don't know what you're doing. First, take care to make sure the area is well-ventilated because of fume risks. Also, be sure to wear eye protection, rubber gloves and clothes that cover your skin completely. Then, pour 5 gallons or so of water in the bowl, followed by the slow, careful addition of 12 ounces of muriatic acid. Next, use a toilet brush with a long handle to carefully spread the solution around the stain. This step might have to be repeated several times over the next hour or two, but doing so should completely eradicate the mark. If you leave the room at any time, take care to close the lid and safeguard that no child or animal can come in contact with the water.
Another last resort option is pumice stone, available at many dollar and hardware stores. Simply use it with plain water to scrub out the stain, but do so very carefully. The harsh stone can damage porcelain irreversibly, and you don't want to trade one unfortunate appearance for another.