Australian Boulder Opal Cabochons, October Birthstone Healing Stone for Rings, Pendants and Bezel Settings

Because it's not cut all the way through the opal seam. The lapidary leaves a layer of the original ironstone rock attached to the back, so what you're seeing is a thin ribbon of opal color sitting on top of natural stone. That's what gives it that landscape like matrix pattern instead of a solid sheet of color.

Not evenly. Some cabochons are heavy on fire, others show a quieter, more subtle line of color through mostly dark matrix. That range is normal for boulder opal and part of why sellers photograph a full batch rather than a single perfect stone, so you're seeing a realistic mix, not a cherry picked sample.

This is a natural, solid boulder opal cabochon, meaning the color layer is part of the original stone with its natural ironstone backing, not a manufactured sandwich of opal, glue, and quartz like a doublet or triplet. If you're comparing listings elsewhere, that's the main thing worth double checking.

The ironstone backing actually works in your favor here. It gives the stone more structural support than a thin sheet of solid opal, so boulder opal generally handles everyday knocks a bit better. It's still an opal though, so it's not immune to chips if it takes a hard hit.

The flat polished back is made for bezel setting, so if you already do basic stone setting work, these will behave like any other cabochon. If you're new to setting stones, a local jeweler can bezel one of these in without much trouble since the shape is straightforward and there's no faceting to worry about.