Accidentally dropping something in the toilet isn't all that uncommon, but when the item you drop is a diamond or other valuable, the situation takes bathroom urgency to a whole new level. Don't panic, and whatever you do, don't flush! Rush in with a quick fix that should get your item back before it ends up in the eternal bowels of the sewer system.
Hopefully, you have a good plunger available, and can quickly jump into action to save the dropped item. Push down gently at the bottom of the bowl, gradually emptying the plunger of air, followed by a rapid, upward jerking motion. You should, essentially, create a system of suction that pulls anything in the toilet trap up into the bowl. The trap is a horizontal gap in the system, designed to keep odors from permeating the room; however, in this case, it could very well be a "safe space" for whatever valuable object was dropped.
Your Hands And A Wire
Depending on the contents of the toilet water, along with how badly you want to retrieve your dropped item, it's worth a try to reach deep into the toilet, slowly feeling around. Long gloves should protect your hands, but if you don't have any, washing with an anti-bacterial soap afterward should suffice. If you can't feel the object with your fingers, bend a coat hanger into a make-shift hook; it should roughly resemble an "L" with a curved angle. Look at the outside of your bowl on the bottom and you should see an outline of the interior piping, so base your coat hanger on that configuration, gently pushing and nudging to (hopefully) find your lost item and drag it to the surface.
Residential Plumbing Services
If you can't plunge the object within reach, nor can you fetch it with your fingers and a coat hanger or other tool, the toilet will have to be removed from its base. Shut the main water valve off, place a big, irrefutable sign on the johnny, advising against use, then contact your local residential plumbing services. While you could attempt to remove the toilet yourself, unless you're well-versed in such a maneuver, things could go horribly awry. Let a plumber not only handle the delicate operation of removing and reinstalling the porcelain perch but of making sure your valuable or otherwise cherished item is successfully retrieved.
Toilet emergencies call for quick and decisive action; however, if the item you've lost in the bowl is of high monetary or sentimental value, stop everything, and call a plumber. That's the safest way to ensure eventual retrieval, with no damage done to the bathroom, and no regrets about where your hands have been.