concretion

Short definition from Wiktionary

Noun

1. 1
   The process of aggregating or coalescing into a mass.
2. 2
   A solid, hard mass formed by a process of aggregation or coalescence.
3. 3
   A rounded mass of a mineral, sometimes found in sedimentary rock or on the ocean floor.
4. 4
   The action of making something concrete or the result of such an action.