Long Term Decorative Item

There should be a decorative block that takes the player a long time to get

I really enjoy the addition of copper in Caves and Cliffs and its oxidation over time showing the age of builds, and I think it would be cool to incorporate a block that takes extremely long periods of in-game time to mature. 

My suggestion would be a crystal that spawns in a rare type of amethyst geode. it can come in any of the 16 colors, and continues to grow in size up to about the height of an armor stand as long as the player is within a certain radius and certain conditions are met. The growth, however, can take 1-3 days of time spent in the game. Due to this amount of time, I feel as though the crystal should be able to be moved and still have the ability to grow. Once obtained, these crystals can be polished, but polishing will stop the growth process. (maybe waxing or brushing?)

In multiplayer, these crystals have three important qualities that would give a lot of potential for economic systems requiring no mods. Not only are they rare, but they take a long time to mature and have no crafting value. They'd make an excellent form of currency.

These crystals could form centerpieces in grand houses, be scattered across fairy villages, and excite visitors from far and wide.