1. Slow Cooling Rate: The most important factor is a slow cooling rate. When a solution or melt cools slowly, the atoms or molecules have more time to arrange themselves into an orderly, repeating pattern, forming larger crystals.

2. Abundance of Material: A large amount of the material being crystallized allows for the growth of larger crystals. More atoms/molecules available means more can join the crystal lattice.

3. Pure Solution/Melt: Impurities can disrupt the regular crystal lattice, leading to smaller, less perfect crystals. A purer solution/melt allows for the formation of larger, more perfect crystals.

4. Space to Grow: Crystals need space to grow. If the solution or melt is crowded, crystals will be smaller.

5. Specific Crystal Growth Conditions: For each type of crystal, there are optimal temperature, pressure, and solution chemistry conditions that promote growth.

Examples:

* Quartz: Forms large crystals in slow-cooling hydrothermal veins deep underground.

* Diamond: Forms large crystals under immense pressure deep within the Earth.

* Salt: Forms large crystals in evaporating pools or lakes where the salt concentration increases and water evaporates slowly.

Let me know if you want more detail on any of these factors!