Trying to get around that awkward "taillées géométriquement":

"...the inlaid pieces, their shape cut to fit the shape defined by the cloisons and polished on the exterior surface, are held in place by a thick bed of mastic."

Or could his "taillées géométriquement" mean something like "precisely cut [to fit]"?

--------------------------------------------------
Note added at 2 days18 hrs (2017-06-26 12:58:24 GMT)
--------------------------------------------------

The problem is that many of the stone pieces (which are not actually "mosaic" tesserai, btw) precisely cut to fit within the defining cloisons are not "geometric" in the English sense of the word.

Moreover, there are a hundred separate pieces of colored "stone" in that illustration, and I am at a loss understanding why (or how) each individual colored element within the cloison frames could be a separate piece of (meticulously) custom cut stone; this technique of cloisonée work is ideally suited for the true enamel medium (i.e., the fields are filled with powdered glass, then baked to liquefy the glass, cooling into a uniform stone-like mass).

But, Bénéditi says that they are "pierres" so, waddayagoinado?

--------------------------------------------------
Note added at 2 days18 hrs (2017-06-26 12:58:45 GMT)
--------------------------------------------------

Suspend disbelief?

--------------------------------------------------
Note added at 3 days17 hrs (2017-06-27 12:26:39 GMT)
--------------------------------------------------

To clarify:

Bénéditi says that what we have here is "pierres" (relatively large and small) which are cut to fit within "cloisons" --usually thin pieces of wire which are "soldered" onto the metal ground to form a kind of "frame" around the colored elements of the design.

As far as I am aware (and it is definitely not my field of expertise), the enamel on cloisonée enamel work is not referred to in French as "pierres" --those elements are made of glass paste which has been melted to form a fused glass, which then fills in the space between the cloisons. It may be that B.'s "pierre" here is in the sense of a stone-like material; but I've never seen such a usage. Check the Grand Robert.

Looking at the object on the digi-ub.uni-heidelberg site, it seems to me *extremely unlikely* that all those dozens of smaller colored elements are (literally) "stones," each one precisely cut to fit the tiny spaces between the cloisons.

The Egyptians bloody well *invented* glass (and they invented enamel work as well, at a very early date), so my guess is that this is what it clearly appears to be: enamel work.

But, Bénéditi doesen't say "émail," he says "pierres," so that's what they were (for him and for anyone else who happens to read him, either in the original or in the skilled translation).

Again, it might be worth checking the Grand Robert (or the Academy's dictionary, which is pretty good on historical meanings which may have fallen into obscurity subsequently).