I'm not an authority, so this may not be accurate. My take is... on that one shown... I'd have said the patch was veneered over the base. Looks like this one has some issue with the expansion/contraction coefficient of the glass and it shows how it was a thin veneer over the base where it separated. I think of veneer in woodworking terms. Like particle board with oak veneer over it. It's a thin layer, usually of a preferred material, bonded over a base of lesser quality or less desired base material. In furniture, you could have a piece made of oak, with the oak being the main part of the piece, but with other contrasting woods veneered over the oak in certain places for decoration and contrast, and to save money by not having to construct the entire piece from the higher quality wood.. much the same as it is in marbles. With wood, it's usually done with an adhesive. With glass, it is melted together, which I would assume occasionally lets some blend with the matrix just like welding metal or any materials that are melted together. This would likely be exaggerated by how hot the two molten materials are when they are introduced to each other. I'm not familiar with how this was done by the marble makers though and I don't know if the term can be applied in marbles the same as it is in furniture. Conquerors, as far as I can tell, are basically a clear marble with a colored patch and some white veneered or somehow laid on the surface to decorate it. Similar to an all-red over a white base or a master with a thick brushed patch. I have some that had the color flake off just like the marble shown a couple posts up. I'll have to pull out the vitro box and look at them closer.
On veneered marbles, does the marble get made and then a veneer is applied over it? Or was the marble made all at once? I never thought about defining what is truly veneered and what is not until now.