In my opinion hardrock require several assays even of the same material. Composition of minerals can tell a good story to the right geologist. f possible find a lab that does small batch. Xrf can also be used.

Placer, it is good to have a purity assay of gold sold this shows production potential as well helps to sell by the gram.

As a prospector, I heavily rely on assays. I sample every lithology change and like mentioned above, numerous samples of the same host to average the %.

1 lb soil samples in ziploc bags, panned down to concentrates, and examined under a 10X loupe or microscope can give you same-day information and cost zero dollars. If the material contains free mill gold you'll likely see it under magnification. You'll also learn more about the country rock, alteration, mineralization, sulfides % and types, indicator minerals and elements, etc.

This information can be used to determine further exploration strategies and tactics e.g. infill soil sampling grids where the samples are assayed.

As CuPorphyryDaddy said above, it's a good idea to sample lithology changes.

The sample bag, pan and loupe may not reveal all, but they reveal much, and within a day, for no cost.

See ARIS reports under the name Eugene Dodd for an interesting heavy mineral concentrate (HMC) sampling method.