Geodes are natural wonders that have taken thousands of years to create. Starting as a rock cavity, perhaps a gas bubble from a lava flow, water over the years has slowly permeated the cavity, depositing its dissolved minerals. This happened slowly enough for the minerals to crystallize into a unique form and color. Deposits of iron and manganese are especially colorful.Each of our beautiful 1 to 2 inch (2.5 cm to 5.0 cm) geodes has been cut open and polished. Collected in Brazil, each is unique and has a void filled with sparkling crystals. Great for gifts for that special student or colleague! Sold in matched sets of two halves. (Note: Even a single half would be appreciated as a gift.)