Your home has a granite counter top, or does it? It was certainly advertised as having one, but is it REALLY granite?
A quick glance at my one counter top tells me it is not actually granite (it is too salt-and-peppery). So, what’s the difference, and are you been cheated with some second rate stone?
Before you wonder if you are being cheated with some second rate stone, it might be good to know the definition according to stonemasons:
Granite is crystalline rock that is harder than marble with large mineral grains.
Seems pretty straightforward right? The rock’s got crystals, is harder than marble, and has large (visible) grains. By that definition, my counter top is granite, but why do I insist it isn’t?
It’s because I am a fastidious geonerd. Before I go over a geologist’s definition, here are some scientific reasons the above definition is less-than-adequate.
- Crystalline rock – I agree, granite is a crystalline rock, but so are all other igneous rocks (rocks formed by cooling molten rock) and metamorphic rocks (rocks warped by heat and pressure). This is a wide swath of rocks, too many to name in a blog article.
- Harder than marble – Using Moh’s hardness scale, Marble has a hardness of about 3-4 (you can scratch it with some glass) while granite has a hardness of 6-7 (you can scratch it with a chunk of quartz). Considering the scale peaks at 9 (diamond) this covers a LOT of minerals.
- Visible grains – In geologic terms, the rock is phaneritic (an igneous rock with visible grains). In my counter top, I see black, white, and light-pink grains, so that makes it phaneritic (an example of an aphanertic rock (grains you can’t see with the naked eye) is a lava flow).
Here’s where the stone mason’s definition and the geologic definition have problems. If you look at terrifying triangle diagram, you will see a bunch of rock and mineral names. All of these rocks fit the stone manson’s definition of granite. They are crystalline, harder than marble, and may have visible grains (depending on the cooling rate of the molten rock).
Any one of these rocks, from diorite to syenite to quartz-rich ganitoid could be your “granite” countertop. So what is the geologic definition of granite?
You can see it in the diagram. It is an intrusive (non-volcanic) igneous rock composed of 20-60% quartz and a high alkali feldspar (pink crystals) to plagioclase feldspar (white crystals); this is around 65 – 90% alkali feldspar.
Here’s how to tell if your counter top is, in fact, granite.
Take a good look at it.
Do you see a lot pink minerals, some clear crystals, and few white minerals? Is so, you probably have a genuine granite counter top. If not, you don’t.
That doesn’t mean you’ve been duped or your counter top needs to be replaced, only that the stone masons definition has clashed with the geological definition, and if you are installing counter tops in your home, I would trust a guy who makes a living cutting and installing stone rather than just a rock nerd.