Natural lava rock, excellent for almost any type of aquarium, terrarium, or reptile tank. Works well in both fresh and saltwater aquariums. Excellent as a base for new coral frag growth or as base rock for a mini-reef aquarium. Pieces are irregularly shaped with a reddish-brown color. May have sharp edges, but they are usually soft and crumbly. Lighter rock, easy to maintain and work with. Each 25lb box of will contain a mix of small, medium, and large sizes. We always give a balanced mix, and you will not receive a box with all small, or all large.