To toss out a proposal of types of stone, I would suggest limestone and shale as good possibilities. Henry Sweets Support the Mark Twain Boyhood Home & Museum at no cost to you! When shopping online, begin your search at http://www.GoodSearch.com, select "Mark Twain Boyhood Home and Museum" as the recipient, and start shopping! A small percentage of your purchase will be donated to the museum at no cost to you! Please share this with your friends and family, and thank you for supporting the museum. May Mark Twain smile upon you! Henry Sweets, Curator Mark Twain Boyhood Home & Museum Hannibal, MO 63401