fire-assay

He saved what he called the "richest" piece of the rock, in order to determine its value by the process called the "fire-assay."

Of course the fire-assay would demonstrate that a ton of such rock would yield hundreds of dollars--and on such assays many an utterly worthless mine was sold.

Bergeron (1991), A re-examination of the determination of Pd, Pt and Rh in rocks by NiS fire-assay followed by graphite furnace atomic absorption measurements.

I have seen men hunt over a pile of nearly worthless quartz for an hour, and at last find a little piece as large as a filbert, which was rich in gold and silver--and this was reserved for a fire-assay!