food grade diatomaceous earth
I very much doubt that all diatomaceous earth is food grade.  For some reasons why, consider the Wikipedia page for diatomaceous earth: "Kieselgur" was discovered when folks in the 1800s were trying to dig a well.  Since knowledge of hygiene was some decades away, it's not implausible that those people may have been trying to dig a well that happened to be near an outhouse.  I'd doubt that diatomaceous earth recovered from a site near an outhouse would be food grade.  

Also, Wikipedia lists "salt-water-derived" diatomaceous earth and specifically mentions that it is:


not suitable for human consumption
not effective as an insecticide
often calcined, which converts the safer amorphous silica present in most natural diatomaceous earths into a far more dangerous crystalline silica.  This can lead to silicosis if long-term exposure to airborne microparticles of crystalline silica occurs.


Thus, any diatomaceous earth that contains lots of salts or which has been calcined is unlikely to be food grade.