Shade variation - Slabs / tiles should be sorted to ensure uniformity of shade. Normally, any colour can be sorted into three shades. Sorting is simple -just lay all the material on the floor and look from different directions.
If you will buy the entire quantity over a period of time,then you should make it a point to know from which specific quarry the material will be supplied as there will always be a difference between quarries.

Grain variation - If grain varies between fine and coarse, such tiles / slabs should also be sorted uniformly by grain size. Sort this the same way as for shade. The further you see it from, the easier it is to sort.

Oiling - Many black granites are light blacks.To make them temporarily look better (blacker), industrial oil is rubbed on it after polishing.This oil evaporates after some months leaving the slab looking very shabby. A burning match will heat an oiled surface and the oil will rub off on your finger.

Colouring - Similarly, red oxide is put on red granites to make them look redder and to fill cracks. To check this, rub the surface hard with your finger and see if some red colour rubs off. by wetting the sawn surface(back side of polished piece), you can see the true colour and nature of any granite.

Colour patches - These are darker or lighter patches or bands of single colours due to mineral localisation; in black this occurs as "oil patches"-patches which look as if oil has been spilled. These are actually permanent patches. Pay less for such materials.
After laying and seeing from different angles, it is easy to see which pieces have these defects. Tapping with your finger nails or spoon may give different sounds on these patches compared to rest of the granite.

Double colour - Sometimes two different grain sizes occur in the same slab, giving the appearance of a double colour. Pay less for such materials.

Cracks - Test by pouring water, wiping and letting dry. Water which goes into a crack evaporates last.

Staining - Many granites stain. Test before buying with wet spices or oil kept on it for a few minutes. Even if staining is not your worry, if you are going to wash the surface frequently, then check how long tap water remains absorbed. Some granites change colour when wet compared with when dry. Since drying is a slow process, the surface looks unattractive during the interim period when part of it is wet and part is dry.

Holes - Some crystallic granites do not have a smooth surface. The surface chips or there are pin-holes. Avoid this type for horizontal applications.

Having decided on a particular granite, it is important to note that shade and grain should be uniform in the entire lot and there should be no colour patches. Do not accept anyone saying that "this is not possible because granite is a natural material". If entire material is from one quarry, it is a simple matter.

Accept defects only to put in hidden places.