Surface Imperfections (markings): On close examination all natural crystals show ridges, grooves, plateaus, pits, and a number of other defects which indicate the disruptions in environmental growth conditions. A study of these features aids in the identification of rough gem stones.
Surface imperfections of crystals include:
- Growth Hillocks: Spiral growths cause low mounds or hillocks on faces which are otherwise nearly flat. Triangles on the base of tourmaline crystals and depressed triangles on the octahedral faces of diamond crystals are very common. Sometimes these become over developed and a parallelism in the structure and orientation of all the faces is seen e.g. Amethyst, Calcite etc.
- Etching and Dissolution: In general, a great many crystals show light to moderate local dissolution, commonly called etching, usually seen as numerous pits, imparting a frostiness to the faces, or as distinct pits scattered on the otherwise smooth face. Close inspection reveals relationships to the underlying atomic structure.
- Thus, the hexagonal pits on beryl itself and their shape are regulated by the beryl structure.
- On the other hand, the boat shaped indentations on the prism faces are not nearly so diagnostic.
- Since every crystal is a neatly balanced structure of like atoms in like positions, it follows that such pits must look the same when present on identical crystal faces.
- For example, in a cubic mineral the same atomic pattern lies beneath all cube faces.
- If one face is partly dissolved and covered with small square pits, all other faces will develop exactly the same kind of pits if attacked by the same solution.
- This is of great importance in identifying like faces when examining etched crystals.
- By turning over a severely corroded specimen, it is sometimes possible to orient the crystals correctly from noting that pits of similar shape appear only in certain directions.
- Oscillatory Striations: Crystals of pyrite, quartz and tourmaline are commonly covered by numerous grooves and striations. In detail, each striation is caused by two narrow crystal faces forming the sides of a ‘vee’. Because several faces are trying to develop simultaneously upon the same crystal area, it is called oscillatory growth. All three species, quartz, pyrite and tourmaline can be identified with a high degree of confidence when these characteristic markings are present on the crystals. Quartz exhibits lateral striations on the prism faces, Tourmaline exhibits deep vertical grooves or striations on the prism faces while pyrite exhibits three sets of parallel striations on adjacent faces.
- Aggregates: Growth of crystals in a confined space results in a crystalline aggregate of interlocked grains without crystal forms. The appearance of these aggregates is very variable from simple, massive to characteristic groups of perfect crystals grown in openings. Remember that any grain of an aggregate, whether developed or a fragment, still remains a crystal with a homogeneous internal structure e.g. Chalcedony, Turquoise, Jade etc.