Not to be mistaken for granite

This is lifted from the book I am currently about to sign off on. U=I hope it will be published under the title What On Earth?

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In 2017, I went to Sri Lanka to look (among other things) at the site of the 2004 Boxing Day tsunami. Stopping at Polonnaruwa, I saw several massive statues of the Buddha, all carved from a local rock, often called a “banded granite”. While I was trained as a botanist, I once studied a bit of geology, and worked with geologists, so I smelt a rat. To my aging botanist’s eye, the rock was gneiss, one of the metamorphic rocks. That meant that it was no sort of granite.

Reclining Buddha carved from local rock, Polonnaruwa, Sri Lanka, with a close-up. 
Yes, it was gneiss.

Some 60 km away by winding road, there is a large rock called Sigiriya, a name derived from the Sanskrit Sīnhāgiri, meaning Lion Rock, rather like Sinhapura, the Sanskrit for Lion City, which is now Singapore. About 1500 years ago, a time when blue body paint was the height of fashion among my ancestors, an advanced Hindu culture established a fortress on top of Sigiriya. As tourists do, we climbed to the top, and so I saw the rock, close up.

Sigiriya Rock, Sri Lanka, but what sort of rock is it?

Sigiriya is built on three layers of gneiss, but many web sites say the stone is granite, and a few assert that it is a volcanic plug—which would make it basalt! I had looked this up before going there, and found that the many contributors to Wikipedia dodge the issue, simply calling Sigiriya “a massive column of rock nearly 200 metres high”. I knew in advance that I needed a closer look. 

Now clearly, ‘granite’, without any doubt an igneous rock, forged and assembled in the planet’s fiery depths, means different things to different people, especially those with little in the way of geological training. It really isn’t rock(et) science, though. 

To a poet, any hard rock is granite, while a stone mason calls any rock with visible crystals granite, but geologists divide those big-crystal rocks up into granite, granodiorite, diorite, gabbro and more. The poet’s granite and the mason’s granite may not be granite at all, and a friend (not a geologist) says he once read that most Australian “granites” are really a relation, granitic porphyry.

On top of Sigiriya rock, I recalled having written a story about the Rosetta Stone while wearing my science journalist hat, some fifteen years before my climb. The tale began with an Australian quiz show causing a bit of fuss and bother after somebody found an apparent error that had been made a year earlier.

The Rosetta Stone: what sort of stone is it? 
Could it be mistaken for granite?

A contestant had said the Rosetta Stone was made of obsidian, but the quizmaster, looking at his cards, ruled that it was in fact basalt. The producers later conceded that it was “really granite”, and said that the contestant should have another chance to win the big money. The three named rocks are quite different, so what was going on? The Rosetta Stone was carved in 196 BCE, and it carries three inscriptions, saying the same thing in Greek, in Egyptian demotic script, and in hieroglyphics.

The content of the three texts is fairly boring, a list of taxes repealed by Ptolemy V, but the use of three languages made the stone very exciting when it was found in 1799 by French forces fighting in Egypt in the Napoleonic Wars. When the French lost a major battle, the stone became a prize of war, handed over to the victors, and placed on display in the British Museum in 1802, where it remains. Nobody seems to have asked the Egyptians what they thought (or think, now) about that… 

The Rosetta Stone, the key to decoding hieroglyphics, was described by its original French finders as ‘une pierre de granite noir’ (‘a stone of black granite’), but these were not geologists speaking. When Egyptologists called it “black granite”, they just meant a dark, fine-grained granodiorite from Aswan. 

A geologist’s granite has large and very obvious crystals of quartz, orthoclase and other minerals like mica. Forming at a great depth, it cools very slowly, leaving enough time for large crystals to develop. Granite is typically 40% quartz (silicon dioxide), and that means it is normally pale in appearance.

Basalt, on the other hand, is black to medium grey, and while it may contain a few larger bits of the minerals olivine and plagioclase, it is typically aphanitic, a term geologists use to indicate igneous rocks in which the grain size is small (less than 0.5 mm), so any grains or crystals cannot be seen with the naked eye.

That leaves us no further forward: why was a piece of granodiorite labelled basalt? Granodiorite is similar to granite, because it has quartz and plagioclase (with no orthoclase), but it also contains biotite and hornblende. It is typically darker than granite, but still a long way from basalt.

British scientists always called the Rosetta Stone basalt, and probably nobody gave the geology much thought, because it was the text cut into the stone that mattered, not the rock itself. When the stone was cleaned in 1998, it was found to be covered with black wax, printer’s ink, used to obtain contact-prints of the inscriptions, finger grease and dirt, with white paint in the incised lettering to make it stand out. 

Calling the blackened stone basalt made some sort of sense, maybe.
Andrew Middleton and Dietrich Klemm saw that the cleaned stone was not basalt at all, and they published their findings in 2003. In chemical terms, they said, the stone is more like tonalite (that lost me as well, so don’t worry!).

If you want to be precise, the Rosetta Stone is made of granodiorite that has probably been exposed to some extra heating. It definitely isn’t basalt, but neither should it be taken for granite—nor mistaken for granite, either.