Oh, I’m so glad to hear it got there after all! And glad to hear you like it!
Well, I’ve been to that island twice the last 4 years(it’s a great place for hiking!) and twice I returned home with 1/4 of my backpack filled with such rocks. That type of rock is called schist and you find them all over Andros. I quite like their shine and they seemed perfect for painting. I took some with me when I moved to the Netherlands with the idea of painting them and sending them to family and friends.