Evening BRI: To answer your basic question, I was an assayer for a bit. special precautions are used to cover just what you are describing to arrive at a relative representative cross section of the area from the sample.

The addition, or lack of one such speciman would naturally unbalance the true assay value of the area, so many samples are added and thoroughly mixed together, quartered, one quarter is mixed again, then quartered once more. the final quarter is then finally ground - 200 400 mesh - and quartered once more fo arrive at the final assay sample.

Two or more portions are then taken out, one for the actual assay, one to be kept as a check assay if needed.

Done properly it comes out surprisingly accurate.

Don Jose de La Mancha