As far as I understand, it is made in a rolling mill operation, in that a slab of steel heated to a red hot working temperature, is run through sets of rollers that squeeze the slab thinner and thinner until it is to the thickness desired. I think that diamond plate would be created by running the steel through sets of rollers with a diamond pattern milled in the upper roller, which would be used when the steel was near the finished thickness.

In a plain strip mill, the same thing occurs with the finished product being a flat sheet of steel, smooth on both sides, and at the end of the line, the strip is coiled and sent for storage, shipment, or further processing such as galvanizing.

GG