They are oxalate based.
Do you know what foods are high in oxalate?
http://www.ohf.org/docs/Oxalate2008.pdf shows you all you wanted to know.
Now the mystery is solved as mom and dad grow pecans.... Yep guess what is high in oxalate... *ding*ding*ding* you guessed it - pecans. Whoops. Guess pecans are out from now on!!