How do open spaces in rocks allow well-formed mineral crystals to grow?

Open spaces in rocks, such as fractures or small cavities known as vugs, provide the necessary environment for minerals to grow into well-formed crystals.123

When mineral-laden fluids—typically hydrothermal solutions—circulate through these openings, they deposit dissolved minerals through chemical precipitation. If a mineral is allowed to grow within a free, unfilled space rather than being constrained by an interlocking fabric of surrounding grains, it can develop its characteristic geometric crystal form without obstruction. This process is responsible for the formation of many spectacular mineral specimens and the crystal linings found within geodes.14