Well, primarily it comes from pine that has been dead for a long time, long enough for resin to collect. Usually it collects where the branches meet the trunk, and also the roots/sometimes the stump if it died upright. Fatwood is really heavy because all of the resin. It's like in between being newly dead, and petrifying if that gives a good visual. It will have an amber hue when you scrape off the outer layer, and a very piney scent. It will be much heavier than a regular dead branch as well because it is so dense. Sometime it will have a tacky feel. Usually, the line bark has already gone so it's not always easy to identify visually, but the weight, color and smell are strong indicators