Tom Joad : Welcome to
Permies.com, Richsoil.com, and a Big Welcome to the Rocket,
Wood, and
Cob Forums ! With 28,000+ Fellow Members Worldwide,
you can come here 24 / 7 and speak to someone who wants to talk about what you want to talk about !
You are right to be a little hesitant to trust the type of clay you have! Usually the Blue grey stuff is an ancient pre-historic ooze very low in Alumina, and most
potters/ceramists,when they find this color out in the field discard it on sight !
However as Glenn was telling you, the tests that determine what is and is not good clay are simple direct and nearly fool proof, I am adding a link to a site I use
for the background knowledge you need to proceed, read this article and how to judge good clay, While this sights for people who want to make high temperature
kilns much of the information in these Articles has helped other Rocketeers a lot !
Link below !
http://www.traditionaloven.com/articles/101/what-is-fire-clay-and-where-to-get-it
for the good of the craft ! Big AL