This cab, which is very large, approximately 1 1/4 x 2 1/2 inches, was not cut (as I usually do) from a large geode or nodule. Rather, the stone, when originally collected by me from The Carver Agate Field, was essentially the same shape and size as pictured.

Rock # 4694

It had the classical amygdaloidal (almond) shape with a flat bottom and rounded top. I hand ground, smoothed and polished the outside of this specimen. While grinding away a small amount of the stone to reveal the underlying sagenitic agate, the crystal filled geode center appeared. When I began grinding, I had no idea that I would grind through the thin surface into an underlying crystal filled center. The end of the stone with the yellow dot which looks like it has ice around it (looks a lot like Antarctica, doesn’t it?) is what I have previously seen and referred to as ‘Swiss cheese’ agate.

Rock #4696

That is not a geological term, only my way of describing the white area around the yellow dot. The yellowish bands are tiny needle-like crystals which form the sagenite. Sagenite often appears as starbursts formed by crystalline mineral growth within a nodule or geode. See photos 749, 789,and 947.

Rock #749

Rock #789

Rock #947

These photos show this starburst quality. The primary specimen featured in this blog has a number of sagenitic starbursts that formed together in a single geode.

Rock #3281

Note the sagenitic rays in the upper left hand corner of this geode which was found in the ‘Deep Pit’ formation of The Carver Agate Field. Click to read more about the Deep Pit formation.

Rock #4696 detail

Sagenite is also a very general term used by rock collectors to describe the neat junk and mineralization that appear captured within many nodules and geodes. An example of this is photo #4699 which looks like a coral reef.

Rock #4699

This is an agate nodule. It is not called a geode because there is no void or hollow within the nodule. The shape of the stone with the rounded top and flat bottom shows that it is a crosscut of the classic almond shaped nodule from The Carver Agate Field in Alpine, Texas. I love this stone.

Rock #4702

Photo #4702 is a slab from a nodule which is almost 75% filled with ‘stray’ sagenitic material with a light-colored fortification agate to the left of the sagenitic material. The technical name for this agate is ‘fragmented filament agate,’ not much discussed in geological circles, but a very common element in a great many agates from The Carver.

How is the fragmented filament agate formed? When the initial hollow bubble in the lava begins to be filled with silica, the inside of the bubble will often have a thin rind, kind of like a bathtub ring, formed by minerals within the lava and the water/silica which enters the bubble. This rind will often scale off the top and sides of the bubble and fall to the bottom of the bubble. This material is then encapsulated by the silica that fills the bubble and creates various agate structures.