|"crab fire agate" is being sold on e-bay and apparently is being sold as a form of agate. It was somewhat controversial because of the use of the name "fire agate", and it is definitely not related to the true material.
I bought 3 - bead/cabochons (photos here) and ground one of the domed sides and found that the external web pattern was more than just a surface coating. This, at least, did not show a simple thin coat on a surface, it was possible the pattern ran throughout the stone and it might be natural, but ...
I then cut one of the stone in-half to see if the web pattern ran throughout the stone. It does not, it is a thick coating around a central solid colored material. It could be that the material comes from a thin seam and that the outer surface is a natural material, but not likely with the outer pattern able to cover the entire surface, even the edges of the finished stone. If it covers the main front and back surfaces, and edges, then the stone would have to be naturally found in its final shape, perfect discs, rectangles, squares and tear drops. Not likely!
In the first image, it appears that there is a fortification in the "agate" with the parallel lines in a typical fortification design. Highlighted in the second image. When the stone is flipped over the fortification disappears and a different pattern is present on the background Again not likely in a roughly 1/4" thick stone.
The second, square stone, has an interesting mark in the center of one side ... more on this in a minute.
The third stone was cut in half with a saw and the cut edges were examined. (closeup 1 & closeup 2) The first closeup shows the solid core center and shows that the pattern runs only about 20% into the stone, but it covers the edges as well as the top and back surface. In the second closeup an edge was ground down a bit on a diamond wheel and show how the pattern disappears as you enter the core material.
I noticed a depressed mark in the center of stone 2, and it looks like it was pressed into the surface. Based on the edge images and this mark, I would guess the starting material is a hot "glob" of material (likely a glass bead) that has a surface texture wrapped around the central color, and is then pressed into a mold to define the shape. Something was likely in the mold that slightly deformed the surface in the second stone.
The hardness and density of the material is consistent with glass. I personally believe that this material is made by deforming glass or ceramic beads into molds.