How Did an Ancient Sumerian Tablet End Up in the Hands of a Native American Chief?

When Chief Joseph, a renowned Nez Perce leader, surrendered to Europeans in 1877, he gave a special gift to General Nelson Appleton Miles. The gift received by the general was a pendant that turned out to be an ancient Mesopotamian cuneiform tablet. How did Chief Joseph come to have this and more importantly, what was it that was written on the cuneiform?