Limestone Pie and Marble Pie are a pair of female Earth ponies from My Little Pony: Friendship is Magic.
They appear as grown mares in this episode when the Apple & Pie families spend Hearth's Warming Eve together. Here, Limestone is depicted as very aggressive, acting as the primary caretaker of the rock farm. She is especially protective of Holder's Boulder, a large sacred boulder on the farm. Marble, on the other hand, is depicted as very shy and soft-spoken, often letting Pinkie speak on her behalf. She doesn't say anything other than "Mm-hmm", a characteristic similar to the equally laconic Big McIntosh, with whom she forms a friendship.
They appear celebrating Hearth's Warming Eve with their and Applejack's families. During "One More Day", Limestone pushes a large gift boulder across the Apple family's living room, and Marble witnesses Big Mac kiss Sugar Belle under the mistletoe, becoming saddened by the sight of it. They later discuss with Pinkie about what gift she should get for Twilight Sparkle. They are last seen during "The True Gift of Gifting", making their traditional Hearth's Warming dolls out of rocks alongside Maud, Mudbriar, and Apple Bloom.