What does Oracle Bone Inscriptions write on? How is it written?

Oracle Bone Inscriptions is mostly carved on the tortoise's bellybutton (a few are carved on the tortoise's carapace) and the shoulder blades of cattle. This was carved with a knife. It takes a long time to carve hands. Divination should be done before carving. First, drill the back of tortoise shell and cattle scapula, and then fire it. According to the cracks on the front after firing, the results can be predicted. Finally, the time, people, content, results and the achievement of the problem are engraved. When lettering, vertical strokes should be carved first, and then horizontal strokes should be carved.