There were actually many types of power grain in the Origin World, but basically all of them were naturally formed. They more some vegetation or fruit which had accumulated enormous vitality.
Another example would be some vicious beast or sea beast's flesh which had attained enormous vitality. These two were seen as power grains by the people of the Origin World.
Therefore, the term power grain was a general term which included all edible life-forms that contained enormous vitality.
All of the above mentioned were naturally formed power grains. In comparison, there were only a scant few types of power grains which could be mass produced.
Croton just happened to be one of the postnatal power grains which had been refined by the natives of the Origin World after countless years of research. It was considered the most popular power grain among the natives as well as having the best quality-price ratio.
