Kidney stone formation is a multi-step process that involves nucleation, aggregation, and crystal growth, with aggregation being a critical step in which microscopic crystals "seed" and clump together to form larger particles.

Aggregation is the subsequent process where these small, individual crystals stick together to form larger aggregates. This is widely considered more critical than simple crystal growth by size alone because growth is often too slow to form a stone that can obstruct the urinary tract during the short transit time of fluid through the kidney tubules.

Retention is the result of aggregation; once crystals form large enough aggregates, they can be retained in the renal tubules or attach to the renal papilla (forming Randall's plaque), where they continue to grow over time into a clinically significant stone.