Odd response from obsidian?

I've been recently trying out some new spells and rituals involving stones, minerals and crystals. Obsidian is a stone I've been wanting to try recently, particularly since it is used in many of the rituals I've come across and it supposedly works well with my Sun sign (Taurus). However, I had very strong physical reactions from two different pieces of obsidian that were in the same bowl at a local esoteric shop. When I held them in my hand (individually), I had the incredibly uncomfortable feeling like my heart was being squeezed and there was a tightness in my chest. As soon as I set the pieces down, the feeling left me. I have had this reaction from other objects in the past (unrelated to magical practises), though I've never been sure why this sort of reaction occurs. Any ideas? I don't want to jump to conclusions. I did end up purchasing one of the pieces, but I've not touched it since.