This is one half of an unusually long Thunderegg all the way from Oregon, USA
A thunderegg is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. Thundereggs are rough spheres, most about the size of a baseball—though they can range from a little more than a centimeter to over a meter across.
Agate is one of the most common materials used in the art of hard-stone carving, and has been recovered at a number of ancient sites, indicating its widespread use in the ancient world.
Agate is a crypto-crystalline variety of silica, chiefly chalcedony, characterized by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks and can be common in certain metamorphic rocks.
160 x 90 x 50mm