For various reasons, I've never found the opportunity to buy them. Now, thanks to Lockwood, I don't have to. O joyous day!
I decided to do a little piece on gabbro in honor of that occasion, but have been sidetracked by this howler:
When magma gets trapped in a single layer underneath the earth's crust, it will gradually cool and form gabbro. Now if the same material were to be pushed up from below under great pressure, it would form granite. But then again, if that magma were to come in contact with water it would turn instantly into basalt. Are you still following me here? Gabbro, granite and basalt are essentially the same material in different forms. In order to form granite, magma needs a good hard squeeze so its component materials can separate and form large crystals. If magma cools very quickly upon contact with air or water, it will form basalt. And if magma is left alone for a couple million years it will form gabbro.
Oh, Paul. Paul, Paul, Paul. Go back to designing kitchens, and attempt geology no more.
He did get this much right: "black granite" is actually gabbro. And he's probably right about the finishes and so forth - I defer to his expertise there, since apartment living has never afforded me the opportunity to install decent countertops. Closest I've been able to come is the current adventure in gluing bits of felt to the undersides of small tiles and laying them on top of the counter, and of course that's just Not Done on surfaces where one wields sharp knives, as things may skid and lead to tragedy. Besides, no grout = maclargehuge mess as various juices and so forth trickle between the tiles. But I digress. We were about to set the poor man straight on the difference between gabbro and granite (and, for that matter, basalt and rhyolite).
The difference, y'see, is rather simple. Gabbro is basalt that cooled slowly deep underground. Granite is basically rhyolite that did the same. Contrary to Paul (who should fire his geology source), cooling by water ain't got nothing to do with it. It's all in the chemical composition, baby, yeah!
Smithsonian Rock and Gem guide, what we have here is a rock composed of calcium plagioclase feldspar and pyroxene, with a bit of olivine and magnetite mixed in. Gabbro is low in silica, quartz is practically non-existent (duh, right?), and it's also low in water. It forms from the same magma that gives us basalt, those nice quiet flowy eruptions that create our sea floors and our Hawaiis. It's not common on the Earth's surface, although it's not exactly rare, either.
One other thing you'll notice in the better class o' rock and mineral guide: granite is listed as felsic, while gabbro is listed as mafic. Felsic basically means it's pale, light, and high in silica. Mafic means it's dark, heavy, and rich in magnesium and iron. There's more to it than that, o' course, but that's close enough for laymen's work.
And my advice to folks who install kitchens: consult a geologist, or at the very least Wikipedia, before you venture an opinion on the geologic origins of the rocks you work with.
Photo sources: gabbro came from here, which has a very easy-to-follow guide to yer basic igneous rocks. Granite came from here. For SIWOTI Syndrome, see here.