Experienced diamond drillers, and certain others, would certainly know the significance of visible gold in core. That would pretty well indicate grades in excess of 1 oz/t.
With respect to the Double Eagle property, the same might be said for the recognition of abundant, coarse grained chalcopyrite in core.
Hence, that could explain why NOT stock has been halted several times recently for only NR about visuals re the core.
There was a story circulating a number of years ago about a junior exploring for Cu/Zn massive sulphide deposits at a field camp on a few claims in a relatively remote area in northern Quebec. The area had not been previously thought to have any VMS potential, hence there was a lot of open ground available for staking. The first few DDH's came back blank, but the company was confident in their assessment of the potential of the property based on their geological, geochemical, etc interpretations and continued drilling. A later DDH hit massive sulphides. The company geologist routinely logged, split and re-boxed the split core at the drill site and had it shipped to an off-site facility a few days later for assaying. Before the samples had even arrived at the analytical lab, dozens of stakers arrived at the remote site and a major staking rush began. Cell phones were still a thing of the future at that time. After checking as best they could for a source for what was considered an obvious leak from someone, it was concluded that it had to be someone who had been involved in the transportation of the split core from the drill site to the lab. No assays were available, the mineralization was relatively fine grained, so what gives? It would appear that someone who had just been handling the core boxes must have noted that some boxes appeared to be considerably heavier than others, and deduced that the increase in weight could only be due to the presence of massive, sulphide mineralization. Unfortunately I don't know if the area ever became a major Cu/Zn mining camp.
