What is a mineraloid?
A geological material that is not actually a mineral but can appear to be.
Mineraloid is a classification given to a naturally occurring inorganic solid material that at first sight can appear to be a mineral but on inspection is found to be outside the definition of a mineral.
A mineraloid differs from a mineral because it is not crystalline and does not have a definite enough chemical and/or physical structure to be classed as a mineral.
Obsidian is an example of a mineraloid, it is hard and rock-like, it can look crystalline and chemically and physically well defined, but is actually a non-crystalline volcanic glass.
It is formed when volcanic material cools too quickly to allow significant crystallization.
Crystallization can cause molecules of the same type to collect together in a well defined structure and with relatively high purity, which often results in a mineral.