When buying a slap, you should be aware of various defects.

Cracks: Cracks can be found in natural granite. You can test this by pouring water, wiping and letting it dry. Water that goes into crack evaporates.

Grain Variation: the tiles and slabs should be sorted uniformly by grain size. The further you look and see it from the easier it is to sort.

Color patches: due to mineral localization there can be darker or lighter patches on granite. In black granite, there can be patches. These are permanent patches, which can look like oil had been spilled on it.

Shade Variation: Laying the material on the floor can do variation- sorting and looking at it from different directions can help you to make better selection.
Double color: some granite can have two different grain sizes in some slabs, given the appearance of a double color.

Oiling: Many black granites are light black o make them temporarily look blacker, industrial oil is rubbed on it after polishing the oil evaporates after some months leaving the slab looking very shabby. Burning a match on the oil surface will rub off the oil on your finger.

Staining: Staining on granite its not something to worry about, if you are going to wash the surface frequently, check how long tap water remains absorbed. Some granite can change color when it is wet. Since granite slowly dries down, this process may take long.

Holes: some crystal granites may not have a smooth surface.
Coloring: by wetting the sawn surface (Black side of the polished piece), you can see the true color and nature of any granite.