Moss Agate

Moss agate has a name that promises two things, and neither is quite true. The moss is not moss: there is no organic matter in it. And the agate is not exactly agate: according to Wikipedia, true agate has concentric bands, and moss agate does not. What it does have is chalcedony, quartz made of very tiny crystals, clear or milky white, with green patterns inside that look just like moss or lichen.

These patterns are called dendrites, from the Greek dendron, tree. According to Wikipedia, they form from minerals, mainly manganese or iron oxides, that branch through the stone like twigs. They are so convincing that, according to Wikipedia, paleontologists have often mistaken them for fossils. A plant that was never alive, and still managed to fool the experts.

The stones in the photo show how different moss agate can be from one stone to the next: one is greyish white with green patches, and another is almost entirely dark green, with a brownish orange patch at one end. According to Wikipedia, the colors come from traces of metals, and the shade also depends on their oxidation state. In Montana moss agate, for example, the red comes from iron oxide and the black from manganese oxide.

According to Geology.com, agate forms when silica-rich groundwater deposits it in cavities inside volcanic rocks. According to Wikipedia, moss agate usually forms from weathered volcanic rocks, and it is found in India, Brazil, Uruguay, central Europe and the United States. In Montana it is collected from the gravels of the Yellowstone River and its tributaries.

It used to be called "Mocha stone" too, after the city of Mocha in Yemen, once one of its sources. Yes, the same Mocha that gave its name to the coffee. So if any stone belongs both in the garden and in a café, this is probably the one.