The big news in the world of Minecraft is undoubtedly the news about the upcoming 1.17 update—named Caves and Cliffs—which will transform the game in a multitude of ways. While the update isn't due to be released until the summer of 2021, we are getting plenty of news about what we can expect in Caves and Cliffs—including the possible edition of rock layers.
As Minecraft exists now, there is no rhyme or reason to the way that diorite, granite, or andesite generate. They simply appear in "splotches" as you're exploring the underground. Last week, someone complained about this to one of the developers—simply stating that they wished those materials generated in a different way. As it turns out, the team had a potential solution—which was sneakily revealed a few days ago.
This isn't the first time that one of Henrik Kniberg's tweets shed a lot of light on what is going to come in 1.17. In fact, we first learned about the "Mesh Caves" from a picture of Kniberg working in his home office. In a similar scenario, we've learned that Kniberg may have taken inspiration from planet Earth's real-world geology and is working on placing diorite, granite, and andesite in "sedimentary" layers in Minecraft.
In the real-world, sedimentary layers form from erosion and (usually) moving water—including glaciers. Imagine a cheeseburger, for example, where the bottom bun is andesite; then water moves in a patty (diorite), then lettuce (granite), so on and so forth. That is how Minecraft's various types of rock will generate in the 1.17 update.
It's a clever and elegant solution to the problem of random splotches of rock when you're digging in your world. Of course, these layers may imply the existence of tectonic plates (which is how granite is formed on Earth), which—oddly—would also imply that Minecraft's worlds are round, not cuboidal.
Without applying real-world logic to Minecraft though, we have to admit that this move makes a lot of sense. We already dig down to certain layers to find diamonds and netherite. It would make sense to be able to find other materials at certain layers as well. It would also make it easier to avoid inadvertently picking up piles and piles of diorite that you don't want. On the other hand, it would also mean that you can collect as much of it as you want.