The finding location of these thundereggs is originated on a small hill above Little Naches River, northwest of Yakima in the Wenatchee national forest. Their matrix is usually greenish. The agate in the center is usually of bluish gray and white hue. Unfortunately many of the stones, which one can find here, are showing cracks and fissures.
The website of the virtual Eibonvale Museum is giving more information and is showing several pictures of specimen from this locality