From the longer Wikipedia page 
Oracle bones (Chinese: 甲骨; pinyin: jiǎgǔ) are pieces of shell or bone, normally from ox scapulae or turtle plastrons, which were used for pyromancy – a form of divination – in ancient China, mainly during the late Shang dynasty. Diviners would submit questions to deities regarding future weather, crop planting, the fortunes of members of the royal family, military endeavors, and other similar topics. These questions were carved onto the bone or shell in oracle bone script using a sharp tool. Intense heat was then applied with a metal rod until the bone or shell cracked due to thermal expansion. The diviner would then interpret the pattern of cracks and write the prognostication upon the piece as well. By the Zhou dynasty, cinnabar ink and brush had become the preferred writing method, resulting in fewer carved inscriptions and often blank oracle bones being unearthed.
The oracle bones bear the earliest known significant corpus of ancient Chinese writing[a] and contain important historical information such as the complete royal genealogy of the Shang dynasty. When they were discovered and deciphered in the early twentieth century, these records confirmed the existence of the Shang, which some scholars had until then doubted.