It can happen quickly -- you catch a flash as something falls into the bowl and then the next moment it is gone as the toilet completes its flush. You may think it is gone forever if you have lost a piece of jewelry down the toilet. The good news is that often the item can be retrieved if you take the right steps quickly.
Shut Down the Water Supply
The first step is to make sure no further flushing or use occurs. The easiest way to do this is to turn off the water supply to the toilet. The supply valve is typically located behind the toilet, so it is easy to access. Once the valve is off, remove the lid from the tank. Use a cup or other small container to scoop out the water from both the tank and the bowl, until there is only a small amount of water left in the bowl. This will prevent further flushes and it will make the following steps easier.
Use a Snake
With luck, the jewelry was heavy enough and the flush weak enough that the item didn't go far. This is highly likely if you have an older model of toilet. You can put on a rubber glove and reach down the toilet drain to see if the item is sitting there just out of reach. Failing that, there are plumber's snakes designed to hook items in drains. It the item is ferrous, which means a magnet can grab it, try a flexible snake with a magnet attached to it. Otherwise, a hooked snake may be needed to attempt grabbing the item.
If you are unable to retrieve the jewelry on your own, then you may need to have the toilet removed. Beneath the toilet in the main waste pipe there is a trap. A plumber can unbolt the toilet from the floor in order to access the trap. In some cases, the plumber may opt to use a drain camera first to verify that the jewelry is actually stuck in the trap. This process of removing the toilet is involved and may take some time. Afterward, the toilet must be properly reseated and bolted back down to ensure it doesn't leak. Unfortunately, if the jewelry isn't in the trap, then it is likely lost for good.
Contact a plumber in your area if you need more assistance with any part of this process.