Which nut is an essential ingredient of French macarons?

A macaron is a sweet meringue-based confection made with egg white, icing sugar, granulated sugar, almond meal, and food coloring. Since the 19th century, a typical Parisian-style macaron is presented with a ganache, buttercream, or jam filling sandwiched between two such cookies, akin to a sandwich cookie. The confection is characterized by a smooth squared top, a ruffled circumference—referred to as the "crown" or "foot" and a flat base. It is mildly moist and easily melts in the mouth. Macarons can be found in a wide variety of flavors.

Source: Wikipedia