WALL ROSARIES, PART IV
In the last article, I showed a lot of pictures of a relatively common type of wall rosary that is made of some sort of ivory-colored material. Until recently I hadn't ever seen one of these in person, and on eBay they are described in wildly varying terminology, so I deduced that most of the people putting these up for sale have no idea what they're made of either.
A frequent guess is bone or ivory. But bone and ivory both have a "grain" of faint vertical stripes -- it's clearer in bone than in ivory. This doesn't, even when a fairly large smooth surface is exposed.
When you actually have it in your hand, this material is also harder and much heavier than bone. A sharp knife doesn't scratch it. On the Mohs scale of mineral hardness (where talc is 1 and a diamond is 10), bone or ivory has a hardness of about 3 to 5, while a steel knife is about 6, so the fact that a knife won't scratch it demonstrates that this material isn't likely to be bone or ivory.
Also, if this material were bone, that would be detectable by a simple burn test. (I bought a broken rosary of this type for tests, so I wouldn't feel bad about ruining a bead or two.) When held to a flame, bone will scorch, but it usually doesn't show a flame -- or if it does, when you remove it from the heat source the flame goes out by itself. The smell of burning bone is also quite distinctive: it's the smell of a dentist's office where someone has been drilling teeth. Bone scorches under the surface as well as on it -- if you rub off the black part, it's brown underneath.
This rosary material, on the other hand, not only shows a flame, it keeps on burning after you remove it from the heat -- you have to actively extinguish it. The surface turns black, and when it flakes off, the part just underneath the surface is completely unscorched, but soft like taffy. And it gives off a strong acrid smell like burning plastic. (This is a hint.)
This material is indeed at least partly plastic -- or "resin," as plastic is often politely called -- but why is it so heavy? A bit of Internet research reveals that some of these rosaries are labeled as being made from "Oxolyte." This turns out to be the key.
There is, as I discovered, a whole class of modern compounds that consist of stone dust with some sort of plastic (resin) binder. Oxolyte is the trademarked name for a particular compound that includes marble dust. There are other variations, which may use marble, limestone or alabaster dust (rarely quartz) as their stone component. These may be called bonded or cultured marble, sculptstone, thermostone, alabasterite or hydrostone. Most of them are from 75% to as much as 90% stone, which is why they are so heavy and hard. They are used for all sorts of ornamental plaques, statues, picture frames, stepping stones and other ornaments, especially for outdoor use since they are fairly weather-resistant. 
These materials can be carved, but their big advantage is that they can be cast in molds, which means a lot of copies of the same thing can be turned out very quickly. Well-made objects that have been cast are often touched up and smoothed, so there may or may not be visible "mold marks" that show where the joints of the mold were. But there's one sign that these objects were indeed cast in molds that is much harder to eradicate -- and the makers often don't bother, especially on surfaces not intended to be seen. Here's the back of the cross of the rosary I bought, and a closeup of the top of one of the beads:
See those round depressions, shaped like half of a small sphere?
Bubbles. From when the casting material didn't completely fill the mold.
 Many thanks to R.V. Dietrich, Professor Emeritus, Central Michigan University for this information.