That is the dimple that forms when gold of a high purity cools quickly. If you cool it slower by heating with a torch you can get concentric circles. I have heard that dimple called many things over the years, both here and in the refineries I visit, but there is no "official" name for it as far as I know.
Maybe now that we have over 40,000 members the GRF has the clout to pick a name and stick with it and the rest of the refining world can follow along.