These are the fossilized shells of an ancient type of oyster called Gryphaea. They spent their lives underwater in soft mud or sand. The curved part of the shell, which is made mostly of calcite, would be buried below the surface with the flat part facing up. The fossils in your bag are from the Jurassic period, which means they were alive about 145-190 million years ago. Most of the fossils you find are partially broken but once in a while you find a really nice complete one with both valves together.
These are sometimes called "Wyoming golf balls". They are a rare type of concretion that formed in the Thermopolis Formation of the Cretaceous. We happen to have a lot of them right here around Cody. They are a little bit of a mystery, but we know that they contain a lot of different minerals. Some of these minerals are Calcium, Iron Phosphate, Manganese, Calcite, and Pyrite. Sometimes Dahllite is found in fossil bone.
CRINOID STEM SEGMENTS
These tiny fossils are parts of larger organisms called Crinoids. The stems and arms contain a "skeleton" which is made up of Ossicles. A stack of ossicles is called a Columnal. They look like plants, but they are actually animals related to starfish and sea urchins. They would filter tiny food particles out of the water. The name of the fossil in your grab bag is Isocrinus, which is a Jurassic crinoid and it was alive about 145-190 million years ago. These fossils are fun to collect, sometimes you lay on the ground and pick them up with tweezers or you can search the outside layer on anthills to see if the ants have been collecting fossils for you. Look at your fossil with a magnifying glass or jeweler's loupe. some pieces are very detailed like the ones in the photo.
Tumbling rocks is a great way to see how beautiful they really can be. Anyone can do it, but it does require a lot of patience. A batch can take one to three months to complete! You can buy rocks to tumble, or find them yourself. Commonly found rocks in Wyoming that polish nicely include Quartz, Jasper, Chert, agate, and petrified wood. You add them to your tumbler with grit and water. The stones are rolled or vibrated constantly for many many days to remove all rough edges and smooth the surfaces. Just like sandpaper on wood, you use finer and finer grit, and then polish until they are as shiny as glass and a pleasure to look at. Polished rocks can be made into jewelry or just enjoyed the way they are.
Some people call these "Squids". The belemnite is an extinct cephlapod like a squid, but they were a little bit different. They had ten arms covered in tiny hooks, which they used to catch small prey. The fossil in your bag is a piece if the animal's internal frame and this bullet shaped part is known as the guard or rostrum. There are many places in the world where there are so many belemnite fossils that people call them "Belemnite battlefields" or "Belemnite graveyards". These are from the Jurassic Sundance formation, which is about 145-190 million years old.
These are rounded nodules of volcanic black glass known as Obsidian. The name comes from a legend of the Apache tribe. In the 1870s, 75 Apache Warriors fought with the U.S. Cavalry on a mountain above Superior, Arizona. When the Apaches were outnumbered and losing the battle, they rode their horses off a cliff to their deaths instead of being killed by the enemy. It was told that as the families of the warriors cried for their loved ones, their tears hit the ground and became these beautiful gemstones.
When pieces of wood or whole trees are buried in mud, sediment, or volcanic ash, there is no oxygen for bacteria to decompose them. water that contains minerals keeps flowing through the sand or mud and eventually the wood itself is replaced by minerals. This process is called Permineralization. Lots of different minerals can replace the wood, it all depends what is dissolved in the water at the time. This is similar to the process that causes other fossils to be preserved. There are places where flooding or landslides have buried entire forests and over time, all of the trees have become petrified. The word petrified comes from the Greek word petro meaning "rock" or "stone" so petrified wood means "Wood turned to stone". You can find pieces of petrified wood all over, from stream and river beds to the tops of mountains. Many times you can see growth rings or "grain" from the original wood.
Selenite is a form of gypsum, along with satin spar and alabaster. Gypsum is calcium sulfate with water, and the chemical formula is CaSO4.2H2O. Gypsum is used to make plaster, wallboard, blackboard chalk, and fertilizer. It is common in the Big Horn Basin of Wyoming and can be deposited from lakes and sea water, hot springs, and volcanic vapors.