Crystal Habit: The relative width, length and the number of faces determine a crystal’s overall shape or habit.
- Since the structure of a crystal determines the size and shape of faces, the habit or general shape is of great importance in identifying many species which prefer to grow in typical habits.
- This can be affected by the conditions of impurities in the solidifying unit.
- A particular habit may thus be characteristic of crystals from a certain locality.
Many terms are used to describe a crystal habit. A gemstone may exhibit both a prismatic and tabular habit, depending upon the conditions of growth.
A crystal is described as:
- Prismatic: If it is markedly longer in the vertical or “c” axis direction than in the other two.
- Some specific terms used to define this habit are:
- Acicular: Needle-like (e.g. crystal inclusions in Rutilated Quartz)
- Barrel-shaped: e.g. Corundum.
- Columnar: e.g. Beryl, Tourmaline.
- Fibrous fine strand-like e.g. Nephrite.
Tabular / Lamellar: refers to a crystal that is much shorter in the vertical or “c” axis direction than in the other two.
Such Crystals can be called:
- Tabular: Thick crystals, e.g. Feldspar, Zoisite, Corundum,
- Platy: Thin crystals, e.g. some Hematite varieties.
- Micaceous: Sheet-like crystals that easily split apart e.g. Mica.
- Foliated: The thin sheets or lamellae may be folded and distorted.
- Bladed: e.g. Kyanite.
In crystals belonging to the cubic system all the faces are equally developed and are called isometric. The majority of crystals lack faces because they grew in groups competing for the same nourishment in the same cavity space. The term massive is used to describe close-unit growths of one or more species.
Familiarity with habits is useful as it enables one to make shrewd and often accurate guesses as to the identity of the crystals. However, it must be remembered that habit implies that presence of characteristic faces, and when the expert examines a crystal, he is also mentally noting what kind of faces are present, how flat and smooth they are, whether they are striated or etched and how they are placed in relation to each other.
Metamict State: Owing to the effects of internal or external radioactivity or similar such natural phenomena, over a long period of time (a few million years), the crystal lattice becomes damaged, weakened and finally collapses due to the weakening and rupturing of the atomic bonds. This gives rise to an almost amorphous state which is known as the metamict state.
In gem stones, this state is found in zircon which has a natural affinity and ability to attract and entrap even tiny particles of radio-active materials like uranium and thorium oxides. These, in turn continuously emit radiations in the form or Alpha, Beta, Gamma rays and also Neutrons. Over a long period of time, these radiations, weaken the atomic bonds and finally cause the lattice to collapse, thus causing the zircon to loose its crystalline structure and turn into a greenish, milky, semi-transparent or translucent, sleepy stone (the concentration of the radioactive oxides may be to the extent of 0.001%). Another stone which exhibits this state is Ekanite.
In the Metamict state as the crystalline form is lost,
- The physical and optical properties like specific gravity, refractive index, birefringence, dispersion, hardness etc. are considerable lowered.
- The metamict state of zircon known as the “low” type has much lower constants than the full crystalline “high” type Zircons.
- All metamict, (low and intermediate type) zircon when heated under specific conditions recover their crystalline form. This destroys the radioactivity and often results in a change of colour of the stone.