According to the literature, shungite is a mineraloid
consisting of more than 98% carbon (Mastarlez et al.
Opal is actually a mineraloid
, defined as an inorganic natural substance with uniform chemical composition but no internal crystal structure; "potch" is a synonym for "common," with connotations of "valueless.
Another Derbyshire specialty in the collection is "Elastic Bitumen" from Castleton, including one huge lump of this intriguing mineraloid
some 30 cm across.
Some mineralogists are reluctant to accept amorphous substances as mineral species because of the difficulty of determining whether the substance is a true chemical compound or a mixture, and the impossibility of characterizing it completely; the term mineraloid
is sometimes applied to such substances.
Rothwell RG (1989) Minerals and mineraloids
in marine sediments, Elsevier.