Blue banded agate nodules or geodes are not uncommon from The Carver Agate Field, but they most always seem to be blue from the outer rind inward to a white euhedral quartz center.

Euhedral quartz are white or transparent visible crystals. The very center of this specimen and the area between the rind and the blue area are examples of euhedral quartz crystals. Usually, these crystals are seen in the center hollow of the cavity in a geode. In blue nodules, which do not have a hollow center or cavity, the center of the nodule, for some reason, is often white.

Rock #4739

In this specimen, however, when the geode initially formed, it had no center–just a crystal lined cavity. Later, the blue colored part of this specimen was formed when a liquid or semi-liquid silica entered the original crystal lined cavity and filled it (almost) with blue banded agate or blue chalcedony. This blue area, when viewed under magnification, has clearly visible banding, AND MORE INTERESTINGLY to me, thousands of tiny dots.

Rock #4739 Detail

These dots seem to be randomly located within the blue area until they reach the outer edge of the tiny euhedral crystalline quartz geode center. Along the boundary of the blue area with the white crystalline center, the dots are lined up like a ‘black ant caravan’. Under magnification, the dots appear as multi-colored tiny spheres, some black, blue, green and even red. I believe these spheres reflect/refract the light entering the stone. The light absorbed versus the light rays reflected back give the blue stone its apparent blue color.

Rock #4730 More Detail

In fact, most of the blue appearing geodes/nodules from The Carver lose the blue color entirely when they are thinly sliced by a diamond saw. Therefore, the blue color you see here is not the color of the blue part of the stone, but just the blue light refracted and reflected by the stone to your eye. As with coloration seen in some opals, I believe these tiny spheres are important in creating the color that your eye is seeing. And one more thing! The saw-toothed outer edge of the blue area of the geode indicates that the blue area formed after the outer euhedral quartz area had crystallized. The crystalline center of this geode shows the same saw-toothed look. So why didn’t the blue area fill in all the rest of the white original geode center? That is a question that is much debated. There are multiple theories, none of which I find convincing. More on this later.

Please note: this posting is also available for future reference in the Carver Reports Section.