Knowing the density of quartz and the density of resin, one could calculate the ratio (by weight) based on the mass of a sample from Cambria or Caesarstone.

Good calculations, OP!

I have not done them but FWIW, I have heard time and again, that quartz countertops are ~80% quartz which is consistent with your calculations.

interesting calcs. Also interesting is discussion I had just this week with rep from a major quartz supplier. I was asking him about this topic specifically due to your assertions that the 70/30 quartz/resin breakdown was wrong.

Quartz rep readily agreed that the amount of quartz actually at the surface was less than the advertised "93%". He also commented that many colors have other inclusions which while harder than resin are softer than quartz (which is very hard). these other inclusions also detract from the effective hardness of the surface. Consider also that there are many quartz colors with few or even zero inclusions such as the solid colors. It is difficult to understand how such colors could be "93%" quartz. Interestingly, the edges on solid colors are much more difficult to finish in the fab shop to an adequate shine. Could this be because a larger proportion of the material is resin which doesn't take a polish very well?

Overall even by your numbers at best 16% or more of the working surface is resin which is only a little harder than jello.

regarding your comment about scratches in quartz surface: "Specialized equipment is required to form a smooth surface on a quartz countertop. Attempts to "buff out" surface marks in-situ using moderately aggressive abrasives are counterproductive, because the resin matrix will be eroded relative to the quartz grains, leaving a network of tiny channels between the quartz grains;"

The "specialized equipment" is an automatic polishing line where the slabs are successively ground by large banks of (18 - 24" dia.) polishing heads with successively finer grits starting around 100 grit and finishing at 1500 or 3000 grit. During the entire polishing process the slab is flooded with water.

The primary reason you can't repair scratches in quartz surface is that to reestablish the gloss or reflectivity of the factory finish after grinding away the scratch considerable heat is required. unfortunately, with quartz countertop material the heat causes the resin to soften and smear. In a shop environment where copious amounts of water can be used it is sometimes possible to effect a repair that will pass if the fabricator is very skilled. At an installed site this is impossible due to the inability to continuously flood the worked area with water.

I can agree that overall quartz surface is a durable material that satisfies many consumers.

I urge you to get hold of a reasonable magnifying glass or loupe (8x magnification is plenty) and look for yourself.

From what I have seen, color can be created using pigments added to the resin and/or a colored stone filler (which may or may not actually be "quartz"). Poor choice of pigments may have given rise to the "fading" tales we have heard, but there are plenty of pigments that will not fade.

The specific quartz patterns/colors we have been looking at are pale/off-white; those do actually seem to be made from quartz. I haven't done any detailed study of the "pseudo-stone" quartz patterns (such as a lot of the Cambria line), because those don't appeal to me aesthetically.

For me, the appeal of quartz is the ability to get a "quiet" near-uniform appearance in pale colors, or a marble look without marble's issues. If I wanted something that looks like granite, I would simply go with granite, because I think granite is a perfectly good material for countertops. In other words, I think the choice between quartz and granite should be driven by purely by appearance/aesthetics, not any alleged functional differences. Personally, I rather like the slightly lower gloss of quartz.

Caleiete63:
"I think the choice between quartz and granite should be driven by purely by appearance/aesthetics, not any alleged functional differences. "

We agree!!! My 1st advice to my customers is always to pick to "look" they want. The attributes of natural stone and quartz surface are different but both materials make excellent countertops (excluding some natural stone like travertine and limestone which IMHO should never be used for tops).

I have some heartburn with the quartz surface industry due to a history of misleading advertising regarding the attributes and performance of natural stone. I can't count the number of times I had to re- educate a consumer about susceptibility to staining, sealing requirements, or radon in natural stone due to misinformation.