The crystals that form inside a geode can be all sorts of minerals. I checked on eBay and I could have bought geodes lined with prasiolite, celestine, calcite, pyrite, barytes or chalcedony though far and away the most common is amethyst, the purple coloured quartz.
A couple of weeks ago we went to see a geode in the now abandoned Mina Rica, Rich Mine, which is in the municipality of Pulpí just on the border between Murcia and Almeria. Between 1840 and 1960 the mine produced iron, lead and a little silver.
This geode is 60 metres underground and it's a little larger than most. In fact it's 8 metres long and just a bit short of 2 metres wide and high. Eight metres is more or less the length of the old London Routemaster RM buses. Inside there are selenite crystals (gypsum to you and me) which are almost transparent. The boast is that you can read a book through the clearest. Most of the crystals are about half a metre but the biggest is over 2 metres long. They are not the largest crystals ever found, that honour belongs to other selenite crystals discovered in an underground cave in Mexico. Those crystals were some 980 metres deep and the space they were in was really hot, too hot for people to bear for more than a few minutes. There was no chance that the cave could be preserved or opened to visitors so it was re-flooded. So Pulpí has the largest crystals that can be visited.Getting to see the Pulpí crystals required a pre booked appointment and a payment of 22€ per person. For that we got to stroll through the old mine and to have a quick look see inside the geode which we did one at a time; left foot on the rock there, balance on that foot, twist your head to the left and there you are. The guide turned on the lights when you were in place and it was impressive. I stayed longer than most but even then that was probably only around 20 seconds of viewing time.
Oh, and just in case you're wondering the Spanish word for octopus is pulpo - Pulpí, pulpo. How droll.
