Slocum Stone appears to be a layered glass of some variety. The top image shows two slocum stones side by side, with very different base colors and fire patterns.

It is harder than opal and much more durable, and does not appear to be affected by heating like true opal.

The color display is similar under clode up, but it tends to look more transparent than the colors in the natural stone.

The bottom image shows an edge view of a slocum stone, purched atop a slice of natural opal. Note that the colors in the natural stone run very much thoughout the stone, where as in the Slocum stone shows strong zoning.

The top (yellowish) layer is very clear and resembles the top of an opal-triplet or doublet. The bottom of the stone (orange-red) is also clear but darker in color. All of the color flash is localized into the contact zone between these two layers. It lloks like the coloring agent may be a metal oxide, or even a glass dye, and it loks like it is added, and then sealed into the stone with the addition of a top.