Grey Rocks or Elephant Skin (or Seegebirge) stones are strongly textured and fit very well into an aquascaped layout. Their natural-looking surfaces make them ideal for larger rock compositions. It's the product of thousands of years of weathering and water erosion that gives it the elephant skin-like texture.
The beauty of this rare stone can really set your aquascape apart from others. Some pieces have natural "dimples" and random craters, perfect for plants to grow on or shrimps to explore.