the size of the stone is what threw me off if it was a 8x2 then i would have bet the house that it was an india stone. but it could be an ark from the size you gave, as to what ark i would say a hard ark i have seen a few older oil staine
d arks with that yellowish color. i dont believe its an washita they have a mixed colors of red purple white grey, i have a few i would like to see better pic's if you want a better I.D some in sun light and some indoors.