When you have Jet....
We can't know, of course, whether jet (the semi-precious stone) had quite this kind of effect on men (or women) in the Renaissance, but it certainly does seem to be regarded as a precious substance and associated with the rich and famous.
As with the distinctive red that represents coral, once you start noticing rosary beads of jet, they are suddenly everywhere in period portraits and paintings. Or at least, there are a lot of black beads in evidence. Some of them may have been a black agate, or black glass, of course; and the less wealthy could have beads of wood or bone dyed black. But if the beads are rich and ostentatious, jet is a good candidate for what they are supposed to be.
For instance, this is a portrait of the late 1520s of someone in the Vom Rhein family, usually thought to be Philipp vom Rhein zum Mohren (1484-1537). It's attributed to Conrad Faber von Creuznach and is in the Metropolitan Museum in New York.
A closeup view:
Jet is actually a variety of coal, and I was a bit surprised (since I know a thing or two about geology) to find out where it stands in the coal "spectrum." Coal is basically organic matter, most often wood, that has been deposited in geological sediments, and then compressed and altered by heat and the weight of more sediments on top. If it's only been altered a little, the coal is soft and brown. The other end of the spectrum is anthracite, coal that has been so compressed and heated that it's become essentially pure carbon and intensely black. As it turns out, the jet used in beads is lignite, a stage somewhere between the two. While anthracite is harder and shinier, apparently it's too brittle to make good beads.
Jet is fairly soft for a gemstone, between 2 and 4 on the Mohs mineral hardness scale, so it can easily be scratched by a knife or coin, sometimes even by a fingernail. Like amber, it is also warm to the touch, and very lightweight, which is a convenient way to tell it from some of its imitations. (Glass and agate are cold to the touch and much heavier.) Good quality jet is hard enough to take a good polish; softer jet tends to be dull.
Here's another black rosary, which I think is also likely to be jet. This is a detail of Agnolo Bronzino's "Portrait of a Lady with a Puppy" (painted in the early 1530s).
Jet seems to have been a favorite decorative material all the way back to prehistoric times, and the Romans valued it highly, as did native Americans in the southwest. The first European source was in Turkey, near a river called Gagat (various spellings) from which the word "jet" is supposed to be derived. Jet is also found in Spain, in China, and most famously in Whitby, England, which has a particularly tough, hard variety of jet that is highly valued. (And since very few people are still mining jet there, about the only true Whitby jet on the market is antique, and priced accordingly.)
On the "prestige scale" of rosary materials, as I mentioned when I wrote about it awhile back in Gauds and Gaudier, jet seems to rank about the same as amber -- above wood, glass, and agate, and below rock crystal, silver and gold.
When I wrote that article, I didn't have any examples that let me judge whether jet was higher or lower in status than coral -- that is, assuming my "rule of thumb" holds true that gauds are almost always of a higher-status material. Now, I wonder whether the beads the Infant Jesus is playing with in Black is the True Colorare supposed to represent jet beads with coral markers -- which would answer the question... at least until I run into the next counter-example!)