Muggles' Guide to Harry Potter/Magic/Gurdyroot

Overview
In appearance, the Gurdyroot looks like a large green onion.

Extended Description
We first see the Gurdyroot when Luna Lovegood produces it from a pocket of her robes while searching for a message for Harry. She hands it to Ron, explaining that it is good for warding off Gulping Plimpies.

At Bill and Fleur's wedding, Xenophilius Lovegood says he has not yet given the happy couple his wedding gift. As he departs, Ron suggests that it might be a lifetime supply of Gurdyroots.

When Harry, Ron, and Hermione are visiting Xeno Lovegood after Christmas in Harry's seventh year, he prepares an infusion of Gurdyroots for them. The resulting tea is a very dark purplish red, like beetroot. Xeno seems to quite like the taste, but Harry finds it undrinkable.

No particular magical properties are ever attributed to Gurdyroot, except for its being able to ward off Gulping Plimpies.