17 October 2008

Sometimes, "granite" isn't granite at all

This is Absolute Black. It is always labeled and sold as granite but it isn't granite. It's a mineral called gabbro.


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.


Why is this important? Well, it's important because those three phases of former magma have different properties. Basalt is the most brittle of the three which is why it doesn't get used as counter top material. Most granites are strong enough to be sliced into 3cm sheets without too much trouble. But gabbro, due to its uniform crystal size and density is stronger than granite. This means that it can handle longer, unsupported overhangs --like on a bar. It can be carved into more ornate edge treatments and since it's so uniform, it is more water resistant. Absolute black is a great stone to make into a sink, whereas you wouldn't want to use most granites for that purpose.

The downside to it (if it's truly a downside) is that it's always black or very nearly black. Chances are good that if you're looking at a uniformly black or very dark grey granite, you're not looking at a piece of granite at all, but rather a piece of gabbro.


Now, due to that uniform crystal size and density, gabbro can be shined up to a mirror-like surface and that's where the problem with it is as it's ordinarily used comes in. It is impossible to keep clean if it's being used as a kitchen counter when it has that mirror finish. I have never heard of someone who had it and liked it. But the problem isn't the material, it's the finish. It will show every finger print, water spot, smear and smudge. It would drive me insane.


Here's a slab of absolute black with a honed finish. The honed finish tones down the black quite a bit, and in so doing, it eliminates the mirror effect.


Here's a whole kitchen done in honed Absolute Black. It's a much calmer counter than the shot at the top of this post, wouldn't you say?

Now, because true Absolute Black "Granite" is a premium, a lot of less-than-honorable stone suppliers will take a less-expensive predominantly black granite and call it Absolute Black because most people can't tell the difference.

Sometimes; similar, dishonorable suppliers will actually dye inexpensive granite with black pigments and sell it as more expensive Absolute Black.

Not that I'd know any of these less-than-honorable business people personally...


Anyhow, if you're in the market for absolute black, make sure that what you're buying is just that. If it looks like this, it ain't absolute black.

And as a completely unrelated yet completely cool side note, the paving stones on the streets of Pompeii are gabbro.


Sometimes I just love geology. Even if the only person I fascinate with this stuff is me.

5 comments:

  1. After months of deliberating countertops, your blogs have helped me narrow my choices. I am currently working with someone I believe to be a reputable salesman but it is nice to be able to ask the right questions and know what the product should look like. Concrete or honed absolute black granite, hmmmm, the jury is out but close to a decision. Thank you Paul.

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  2. Thanks! That's the best news I've heard all day. If my daily ravings and rants can make you a better educated consumer than I'm doing my job. Not that you asked, but go for the honed absolute black!

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  3. I'm partial to engineered stone tops that look like honed absolute black, but (a) come with warranties and (b) are non-porous.

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  4. Honed Absolute Black does seem like a fingerprint factory when I see it at the wholesalers, but I've never actually seen it used anywhere. Ming, my first commenter here used it. I wonder if I could get her to write a quickie review of how its wearing.

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  5. great post. who knew? thanks for the info.!

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