Montana Dendrite Agateis a creamy variation of chalcedony from the alluvial gravels around Yellowstone River. It was formed during the Pleistocene period. Eastern Montana was largely a shallow sea, with massive woods surrounding its borders and islands of volcanoes pouring lava on parts of the forest. Thousands of acres of trees, redwoods and sequoias, were ravaged for hundreds of miles around. Generations of trees sprung up to be destroyed again and again by the hot lava. When the period of volcanoes and lava was through and the rains arrived, rich mineral silica-water carrying ash debris poured into the earth, filling voids and cavities in the igneous host rock. This is how this beautiful stone was born. There are no cracks in a high-quality Montana Agate. It is typically clear to pale yellow or greyish-blue, translucent and banded with detailed tree-like plumes of iron oxide (red) and manganese oxide (black) inclusions. Montana Agates have a hardness of 7 on the Moh’s scale. They have been used by Native Americans for arrowheads, and they are popular in the jewelry trade since the early 1900s.