This kind of stone has bubbles on the surface, which is very hard. After being broken, there are cracks inside, and there are transparent crystals. How did this come about? What crystals are there?

This is a kind of stony nodule, which often contains some mineral lumps with obviously different composition from the surrounding rocks in sedimentary rocks, and is called nodule. Its shapes are spherical, oval, lenticular and irregular. Its internal structure is concentric and radial. Its size ranges from a few centimeters to dozens of centimeters or even meters. Some are layered, and some are beaded along the layer. According to the reasons, it can be divided into:

(1) Primary nodules refer to nodules formed by the accumulation of certain minerals or chemical components around other substances during the deposition process. For example, limestone contains flint nodules, sandstone contains iron nodules, in addition to pyrite, siderite, apatite and other nodules, and there are a large number of iron-manganese nodules on the modern seabed. Such nodules generally do not pass through the bedding.

(2) Epigenetic nodules refer to the nodules formed after the formation of rocks, which are leached and infiltrated by mineral-containing solutions, precipitated around some centers, or metasomatism with some substances in rocks. It is characterized by tuberculosis passing through bedding. Lime nodules in loess belong to epigenetic nodules.

As a result of weathering, a nodular mass can be formed, which is called pseudotuberculosis. For example, some rocks are divided into many geometric small pieces by vertical and horizontal cracks, and then the solution containing iron hydroxide permeates along the cracks to dissolve the rocks, and finally the divided rock small pieces become pseudo-nodules surrounded by limonite.