In order for people to define rock to be mined as ore, they have to know the amount of the metal of interest contained in the rock.  If there is enough of that rock, and if there is enough metal that can be recovered by processing and that can be sold at a price greater than the expense to produce that metal, then the rock can be defined as ore to be mined.  The assayer determines the amount of the metal in the rock.

The method used to measure the amount of metal (or other commodity) of interest  is called assaying.  The most common and reliable method for assaying rock for silver, gold or other precious metals (platinum group elements) is fire assaying.  This method was used in the Virginia City mines and is still used today.