Crystal

The word crystal is used in two distinct ways in mineralogy, and the difference between them matters more than it might first appear. ^[1]

Restricted Sense

In strictly constrained usage, the term designates a mineral fragment enclosed - wholly or partially - by planar, euhedral surfaces that developed naturally during its unobstructed formation. ^[1] The presence of those faces is not cosmetic - it is a direct consequence of the fact that atoms inside the mineral are arranged in a regular and repeating manner. ^[1] Crystal faces form because atoms at the surface of a growing mineral organise themselves according to the same internal lattice that governs the whole structure. The flat, geometrically regular faces you see on a well-formed quartz or pyrite crystal are, in this sense, the macroscopic signature of atomic-scale order.

Broader Sense

More loosely, geologists frequently adapt the label to describe any individual mineral fragment, without demanding planar morphological perfection. ^[1] Under this relaxed definition, the specimen’s exterior envelope might consist of true growth faces, jagged fracture planes, distinct cleavages, or simply the interfering boundaries shared with adjacent crystalline material. ^[1] Most minerals in rocks do not have well-developed crystal faces - they grew in contact with neighbouring minerals and their shapes were determined by competition for space, not by free growth. Calling those grains “crystals” in the broader sense is common, but it strips the word of its more precise meaning.

Which to Use

The cleaner approach is to restrict “crystal” to instances exhibiting actual euhedral development, defaulting to “mineral grain” whenever the specimen’s boundary morphologies are mixed, damaged, or uncharacterised. ^[1] The reader should be aware that the literature uses both senses, often without signalling which is intended. ^[1]

References

1. Nesse, W. D. (2017). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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