Chiastolite (aka Andalusite, aka Cross Stone, aka Fairy Cross) has a orthorombic crystal structure and is in the Aluminum Silicate family. This is one of the stones on the “Do not use in an Elixer” list because of the aluminum in its make up. It is generally formed near granite deposits and is quite plain, and a greenish color on the outside. I first encountered this stone in Fresno, California where it is known as the Faeire Cross (and I honestly have no idea why, it just is). On the east coast of Maine, it is simply known as the Cross Stone and can be found in granite boulders that have been cleared and placed in “farmer’s walls” that separate fields. Be sure to ask before you go tapping with your hammer if you go exploring.
So where do the cross patterns come from? The variety of Chiastolite, a form of Andalusite, has formed in cigar shaped bulges that have wrapped around Schist (an altered, super reheated mica with some graphite, almost black looking). Again, plain on the outside with a green to tan coloring. When these cigar shapes are sliced, the cross pattern appears. Depending on the size of the crystal found, full square patterns have been revealed in crystals that are 3 to 4 inches in their girth. Generally though, most is found in a 1 to 2 inch size, and only the cross pattern is revealed.