Dragon stone also known as Ohko stone.
Dragon stones are presumed to be igneous rocks that have been extensively eroded from water. It’s been said that Ohko stones are just like hardened clay that has the additional unique texture and ripples to its shape.
These stones are pH neutral and should not have an effect on the water in your aquarium. That said they often have lots of dirt and clay deep within the holes of the rock that may lower your Ph. A good cleaning and soaking will be required if you are concerned about your water chemistry. But for those of us with planted tanks, clay deep inside the rock should make great places for epiphytic plants to root.
Larger pieces can be broken in to smaller pieces with a hammer and chisel if desired. One of the most popular aquascaping materials available, especially for smaller tanks.
We recommend 2 pounds of stone for every gallon of water your tank holds. A 5 gallon tank would need 10 pounds, 10 gallon requires 20 pounds etc. Of course this is just a recommendation, the amount of stone needed will depend on your plans. It's always nice to have a bit extra for more layout options too!