Bev, a number of years ago Tvo posted some stuff about hypertufa. You make
it from various dried vegetation, like chopped straw or peat, cement, and
sand. It forms a semi-porous rock-like material. You can make hypertufa in
any form you want. I have been contemplating re-making my raised beds with
hypertufa sides. I think it is realy cool looking. Here is a link. Google
hypertufa to find lots more.