On a visit to Tucson, relatives took me to a gem and mineral show where we saw a collection of geodes. Ordinary on the outside; beautiful on the inside. Will you please tell us how they are formed and ...

Geodes are irresistible. They fill display cases and museums everywhere. I’ve even got one on my desk at home: a smooth, brownish rock encasing milky lavender crystals. It’s hard to deny the allure of ...