Gabbro —

is dense, greenish or dark-colored and contains pyroxene, plagioclase, amphibole, and olivine (olivine gabbro when olivine is present in a large amount).

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. 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.