The Excalbians are a sentient species. They are a rock-based species.
They can look two different ways. One is to look like an ordinary immobile rock. The other is to appear humanoid but rock shaped. They have claw-like appendages for arms.
In 2269, they used illusions to draw other species to their homeworld. They did this to learn more about the values of other species.
One member, Yarnek, was a scientist. This scientist was able to make a Class-M environment on an inhospitable planet within moments. Yarnek was also able to take contol of objects. He took control of the U.S.S. Enterprise in 2269.
Captain James T. Kirk and Commander Spock were forced to take part in the the experiments. Yarnek decided the results of the experiments were inconclusive and decided he would continue to experiment on other species.