What is a gel in mineralogy ?

Gel : definition

A gel is a substance resulting from the solidification of colloidal substances (very fine particles). Gels usually consist of uncrystallized spherical colloidal agglomerates (flocculates). Despite the absence of a crystal lattice, some of them, such as opal, are nevertheless considered to be minerals. Often unstable, these gels frequently recrystallize into cryptocrystalline minerals.