If you visit a natural museum to see the mineral exhibition, you will find that crystals often grow in hollow rock caves, just like an elaborately carved crystal house. This kind of rock caves filled with crystals is called a crystal cave. The original space of the crystal cave is generally formed by volume contraction during the cooling of magma, or it may be the result of the escape of a large amount of gas during the solidification of magma, or the result of the local dissolution of rocks leaving a void. An open free space is a necessary condition for the growth of crystals, but just having space is not enough. As the saying goes, "even a talented cook cannot make a meal without rice". Mineral growth also requires a certain material basis. After the formation of the crystal cave, if ore-bearing hydrothermal fluids are injected into it, when the temperature and pressure drop and cause the solution to be supersaturated, corresponding mineral crystals will gradually crystallize and precipitate.
When high-temperature silica-containing hydrothermal fluids inject into the crystal cave, if the crystal cave itself is relatively cold and the temperature difference with the hydrothermal fluid is large, then the initial supersaturation of the mineral solution at the edge of the crystal cave will be very large, and it is easy to form many small "nuclei" in a short time and allow them to grow stably and continuously. It can be imagined that so many small nuclei growing along the wall of the cave at the same time will soon touch each other and cannot continue to grow. Therefore, the earliest formed and located at the edge of the crystal cave are often agates - aggregates of cryptocrystalline quartz. As the crystallization process gradually moves towards the center of the crystal cave, the temperature gradient and supersaturation decrease, and the crystallization rate slows down relatively. At this time, it is particularly suitable for the continuous growth of large nuclei, and the resulting crystal crystals are cone-headed, hexagonal columns. Because each crystal has shiny crystal faces at different angles, the reflection of these small crystal faces in the crystal cave under light illumination is like the twinkling stars in the night sky, which is very beautiful.
In the same crystal cave, the size of the crystals grown at different development stages varies significantly. If the ore-bearing hydrothermal fluids still contain a trace of iron ions, when the iron ions enter the crystal lattice, the crystal will show a noble and beautiful violet color, such as the amethyst caves often seen in Brazil and Uruguay on the market. As the crystals continue to grow, the concentration of ore-bearing substances in the hydrothermal fluids gradually decreases until it is exhausted. Without the "nutrient supply" of the crystals, the growth will naturally stop. Therefore, the center of the crystal cave is often empty. The original form of the crystal cave is a hollow sphere or ellipsoid, which is cut in half after mining. In this way, we can clearly see the beautiful crystals hidden in the crystal cave, and at the same time, we can understand the formation process of the crystal cave according to the observed layered structure.