Elephant Skin Stone is a sedimentary rock composed of mostly calcium magnesium carbonate and is commonly called Dolomite Rock, Dolomitic Limestone, and Dolostone. 
It is very heavy and comes in various shades of gray or reddish-brown, the Oxidized iron and magnesium make the rock appear browner or reddish-brown.
It increases the pH of water making it alkaline.
It is a hands-down choice by aquascapers, as it makes amazing background scapes and centerpieces.