This collection consists of 17 Geode halves. Geodes are secondary rock formations fairly common in sedimentary and volcanic rocks. Many of the geodes are purple and blue in color, suggesting that they have a Quartz (a tectosilicate mineral) composition. One of the geodes is a rusty color. They are of variable sizes, with the smallest measuring approximately 6 cm x 5 cm x 4 cm in size, and the two largest measuring 18 cm x 13 cm x 8 cm and 26 cm x 11 cm x 2 cm in size.