I read somewhere the other day about someone who had a crystal ball which caused a fire when the sun´s rays struck it. This reminded me of a similar thing which happened here at home a few years ago.
I don´t have a crystal ball but I do have a glass one with air bubbles trapped inside it which I bought on my travels. It is quite heavy and I use it as a door stop. Anyway, at that time I had a cleaner who helped look after my father. She was doing my room and put the ball on the bed while she did the floor, then went off to see to lunch for my dad. A couple of hours later when she returned to my room the ball, which had been sitting in direct sunlight, had concentrated the rays and actually burnt a hole right through my duvet cover. Thank goodness she found it when she did otherwise my bed might have caught fire. I was at work at the time.
Ever since then I have been extremely careful not to leave anything, including eyeglasses, where the sun shines through the windows.