Some fossils are rarer than others...

 

Here is one of the more uncommon fossils from the Sandhills Habitat in Santa Cruz; the fossilized upper and lower claw of a crab from the Santa Margarita Formation rock layer. This is 10-12 million years old and I've only seen one of these in over 4 decades. The reason something like this isn't that common is because the Santa Margarita rock layer generally represents ancient higher energy environments consisting of areas of the ocean where course sands and gravels are deposited.