If it's just a dozen or two edges you want to dull, there are a variety of ways that you can do this:
When I was installing ceramic WALL tiles, I'd just use a sanding block, and that was good enough to sand down the very hard glazed surface of the tile, albeit slowly. I think that if you used one of the high tech sandpapers by Norton or 3M on the market, that'd probably work fine.
But, they sell rubbing stones specifically for dulling the cut edges of ceramic floor tiles:
Or, you can use a tungsten carbide grit file made specifically for this kind of work:
The most expensive option, but probably the most efficient would be a diamond grit file:
If you want to do a lot more than a few dozen, then I'd be inclined to put a metal or masonary cutting blade in a hand grinder or use a belt sander. If you do that, make sure the wheel you use is rated for the speed your grinder spins. Also, in this case it wouldn't matter whether you use a wheel for metal or masonary. Typically, wheels meant for cutting metals will be made of aluminum oxide, whereas those meant for cutting masonary will be made of silicon carbide. The reason for using these two different abrasives is that silicon carbide is harder than aluminum oxide, so it makes for a longer lasting wheel. The problem is that carbon atoms from a silicon carbide cutting wheel can enter the metal the wheel is cutting, and change the composition of the metal near the cut, making it harder but more brittle. And, that affects everything; from machining that harder more brittle metal to welding it. If you're using a hand grinder to grind your ceramic tiles, then that reason isn't applicable to your situation, so either kind of wheel would work fine. (Silicon carbide will typically provide a longer lasting wheel because it's a harder abrasive.) Any place that caters to the welding trade will sell 4 and 5 inch metal cutting wheels rated for the 10,000+ rpm speeds that hand grinders typically spin at.