The skin only appears on secondary gravel jade—such as mountain jade, black sandstone jade, semi-mountain-semi-water jade, and water jade—while primary rock jade does not have a skin. Gravel jade originates from the transformation of rocks; it is during this complex and protracted evolutionary process that jadeite develops its skin. The mineral composition of the skin reflects the mechanism by which it formed.

Research has shown that the mineral composition of jadeite skin includes minerals such as montmorillonite, halloysite, gibbsite, chlorite, and serpentine. These minerals form the skin of different types of jadeite:

Mineral composition of white sandstone: gibbsite;

Yellow and white sandy skin: gibbsite and kaolinite;

Yellow sandstone and red sandstone: illite and montmorillonite;

Black skin shell: Kowenite;

Wax-like shell: Lizardite (or containing chlorite).

These secondary minerals are all formed during the “pericarp phase”—the epigenetic geological stage—in which jadeite grows. During this period, affected by crustal movements, jadeite finds itself in an epigenetic environment. Weathering causes it to break away from its parent rock, forming transported stones (gravel). Later, due to prolonged burial, the jadeite mineral on its surface undergoes hydrolysis, giving rise to new secondary minerals that alter jadeite’s original appearance. As a result, jadeite acquires a completely transformed outer shell.

Due to the differences in the surficial geological environment and burial depth of jadeite gravel, the types of crusts formed—namely, the secondary mineral assemblages—also vary.

When jadeite pebbles are buried relatively shallowly beneath overlying sediments and remain in an oxidizing environment above the groundwater table, the aqueous solution becomes acidic and is rich in free oxygen, which greatly promotes oxidation. As a result, the jadeite mineral on the surface of the pebbles undergoes hydrolysis, releasing sodium (a base) into the solution and causing some of the silica to dissolve into the solution, thereby forming a colloidal clay mineral—montmorillonite. Once all the sodium (a base) has been leached away and additional silica continues to dissolve, the previously formed montmorillonite is further broken down, giving rise to a clay mineral—halloysite—that no longer contains sodium (a base). If the jadeite mineral is completely decomposed and all its silica is fully released, a stable mineral—gibbsite—is then formed.

Since these secondary minerals are predominantly white, they give rise to light-colored rinds such as white sand skin or yellowish-white sand skin. In contrast, darker rinds like red sand skin are caused by the impregnation of soil color—specifically, the inclusion of trivalent iron elements.

As the jadeite gravel becomes increasingly buried beneath overlying sediments—reaching depths of hundreds of meters or even over a kilometer—and finds itself in a reducing environment below the groundwater table, its aqueous solution gradually shifts from acidic to alkaline due to the progressive depletion of free oxygen. Moreover, the temperature and pressure in this environment exceed normal ambient conditions, giving rise to low-temperature hydrothermal activity. Under these circumstances, the jadeite minerals on the surface of the jade not only undergo hydrolysis and hydration but also experience metasomatism. With the participation of magnesium and iron elements, secondary silicate minerals—such as clinochlore—begin to form. Since clinochlore exhibits colors ranging from green, green-brown, gray-green, green-black, gray-black, to black, and its color deepens with increasing concentrations of divalent iron, it gives rise to a dark-gray to black outer crust known as "wusha," or "black skin."

If the chlorite formed under relatively high temperatures and with the participation of extraneous substances (such as magnesium), a component called soapstone will appear in it. Soapstone is a subspecies of talc, characterized by its smooth, slippery texture; consequently, the resulting crust is referred to as a "slippery crust."

The serpentine within the wax-like rind is a product of hydrothermal alteration. It forms because the jadeite pebbles were buried relatively deeply, leading to increased geothermal temperatures and the involvement of substantial amounts of external (magnesium) elements. Since serpentine forms relatively late in the process, the wax-like rind typically adheres to the earlier-formed sand-like rind. Consequently, jadeite may exhibit not only a single wax-like rind but also multiple layers of rinds, including the wax-like rind itself.