Flint was never a molten rock. Flint is part of a sedimentary rock such as chalk or limestone and was laid down in warm seas millions of years ago - in the case of chalk around 60 - 120 million years ago. The chalk is calcium carbonate and is largely the deposits of the chalk (CaCO3) skeletons of microscopic organisms called foraminifera. Other microscopic organisms called diatoms contained silica (SiO2). Sponges in the sea floor also included silica in their structure.
Over millions of years, and due to groundwater, the silica molecules have come together as concretions - often around a fossil sponge skeleton. This is similar to the way in which crystals grow but in this case the structure is crypto-crystalline. True crystals fracture along cleavage planes but flint has a characteristic conchoidal fracture.



