The natural capital can be evaluated from different points of view. One of them, and perhaps the most commonly known is the economic point of view. Nevertheless, standard economy is only concerned with what which being directly useful to man, is also acquirable, valuable and produce-able. For this reason, most of the natural resources, remain outside the object of analysis of the economic system. The price-fixing mechanisms, rarely take into account the concrete physical characteristics which make them valuable.

But natural capital has at least two physical features which make minerals or fresh water for example unusual: a particular composition which differentiates them from the surrounding environment, and a distribution which places them in a specific concentration. These intrinsic properties, can be in fact evaluated from a thermodynamic point of view in terms of exergy (Valero et. al. 2003 , Göran Wall 2003).

The decrease of mineral capital comes from two interrelated factors. The first and more evident one is that as demand increases, more mineral has to be extracted from the mine and hence the total available quantity decreases. The second one is that continuous extraction implies declining ore grades, meaning higher quantities of ore needed to be processed and thus more energy. The work needed to separate a substance from a mixture does not follow a linear behavior with its concentration. On the contrary, the Second Law of thermodynamics dictates that the effort required to separate the mineral from the mine follows a negative logarithmic pattern with its ore grade. 

Hence quantifying the decrease of mineral capital in terms of mass is not enough since it does not take into account the quality of the minerals in the mine. However the exergy indicator can overcome this deficiency because it can evaluate at the same time, composition, concentration and of course quantity, by multiplying the unitary exergies with the tonnes of the resource considered. Furthermore, exergy is an additive property within resource accounting and can be used as a global natural capital indicator.

The difference between the concentration exergies obtained with the ore grade of the mine (xm) and with the average concentration in the earth crust (xc) is the minimum energy that nature had to spend to bring the minerals from the conditions in the reference state to the conditions in the mine. The concentration exergy of a mineral is not proportional to its production rate. In this case, bc increases also as the ore grade decreases.