Always capitalize the first word of a sentence! (Actually, that is an appropriate exclamation mark. 🙂 It shows how frustrated the copyeditor is that it’s ever necessary to explain such a basic rule of writing.)
Change 10 to ten.
Sigil is not the right word in this context; a stone is an object, not a symbol. Try talisman instead. (And if you don’t know what a word means, either look it up or don’t use it.)
Delete the exclamation mark after magic; the sentence isn’t finished yet.
(*is too sick of this example to keep explaining all the corrections*)
Here’s the corrected version:
When a child turns ten, they are given a talisman: a stone that, once placed on something, will grant the owner magic based on that object’s material or function. Then the stone will disappear forever. You, however, dropped your stone somewhere in your house, activating it, and ended up with the most laughable magic ever. That didn’t stop you, though. Years later, you’ve become a high-ranking wizard, respected by many and laughed at only by the foolish.