Granite, basalt or porphyry: how to tell what you are actually being quoted

Most quotes for “black granite” are not quoting granite. Hainan Black, Mongolia Black, Zhanjiang Black, Zhangpu Black, Fuding Black and Vietnam Black are basalt or dolerite — a different rock, sold under the same colour name.

That is not a problem in itself. It becomes a problem when the specification, the approved sample and the invoice describe three different things.

Three families, one block selection

Granite is a felsic intrusive igneous rock. The magma cooled slowly at depth, which is why you can see the crystals with the naked eye. Quartz, feldspar and mica make up almost all of it. Hardness is above 6 on the Mohs scale, and density runs about 2.70 for white, light grey, red and yellow; 2.85 for dark grey; 2.90 to 3.00 for black.

Basalt and dolerite are fine-grained, darker and slightly denser. They take the same finishes and are quarried and cut alongside granite. For paving, kerbs and cladding the working behaviour is close enough that most jobs do not need to split the order.

Porphyry has large crystals set in a fine-grained matrix. That contrast is what gives it the speckled face and the wear resistance that makes it popular for heavy-traffic paving.

Where the distinction earns its keep

Two places it shows up on a real order:

• Weight per container. Density decides how many square metres fit in a 20FT. Dolerite at 2.95 against a light grey granite at 2.70 is roughly a 9% swing. If you are loading to a weight limit rather than a volume limit, that is a whole pallet.
• Colour matching across a container. The batch you approve as a sample and the batch that gets cut three weeks later are not the same block. Someone has to look at both and decide whether they hold together in daylight. That call is judgement, not a certificate.

What to put on your enquiry

Tell us the sample name you already specify, and the market you are loading to. If you only have a photo, send the photo — we will tell you which family it is likely to be, and whether the block we would cut it from will hold the colour.

We will also tell you when the answer is that we cannot hold it. That is cheaper for both of us than finding out at the port.