Diamond drill core samples usually end up with a transverse measure (estimation) of gold expressed as grams per tonne of ore, or more typically (but slightly misleadingly) as "grams", given by physically looking at a slice and using surface area of gold to surface area of rock. It is not measured in g per cubic metre.
The usual way they express this is along the lines of "5.3 g Au at 12.6 metres" where the 12.6 metres is how far down the core sample the measurement slice was taken (ie depth). Having several diferent "g" @ several different depths simply is a measure of the size of the orebody both along individual core samples, and using many diamond drill core samples from a grid pattern on the ground.
As an aside, obviously there is a point at which the gold is not economic to mine, and there is usually a statement by the geologists of what point they call the "edge" of the minable orebody is, typically about 0.2g/tonne
