Based on the Spanish legend: if you've been a good child Santa will give you a gift, if you've been nasty child, Santa will give you a lump of coal. This is a typical gift for badly behaved children, a black coal shaped candy lump.
Package made of sugar candy which contains a jewel. A sweet piece of coal, to give as a present for Christmas. It disguising It´s nature becoming a playful packaging by presenting the wrong appearence and hiding the real surprise: "have you been bad? ... or have you been good?".
By separating It´s two parts of coal shows the true gift: a jewel."You have been good."