Define "large". It is hard to tell the size of the specimen from the photo. If it is over 12" the specimen is probably from Southern Brazil, state of Rio Grande do Sul. I have seen amethyst from there with calcite crystals before, but never filling the geode so completely. There is a possibility the calcite formed after the amethyst had finished its growth, and in that case the amethyst would be terminated and lustrous, but of unknown quality. Removing the calcite would involve using muriatic acid (pool cleaning acid), which is HCl (hydrochloric acid). I personally would not remove it since it is a more interesting specimen with the calcite than it would be without it. I hope this answer is some help.