Major Physical Properties of Gems

Although there are a dozen or more physical properties which can be measured, in this course we will concentrate on just a few. In particular, our focus will be on those which are either visible directly, or measurable with minimal equipment, and those which are most important as indicators of a gem's identity, and/or its suitability for particular uses:

Cleavage: In the three dimensional structure of certain crystals, atoms are bound more tightly to each other in some directions and more loosely in others. As a consequence, when strong forces are applied, relatively clean breaks may occur in these "weakest link" directions. These breaks, which can sometimes be so smooth as to appear to have been polished, are called cleavages. The number of directions in which a particular material cleaves, the ease with which that happens, and the "perfection" of the breaks are used to quantify this characteristic.

Since cleavage, or lack of it, is a species trait, it also serves as a good gem identification criterion. In the examples below, the number and completeness of cleavage of three species are shown.

Species with easy or perfect cleavage, particularly when such is the case in multiple directions, are poor risks for most jewelry applications. Not all gems show cleavage however, for example tourmalines, sapphires, and garnets do not.

[Apatite: two, imperfect (note that cleaved surfaces are somewhat rounded and irregular); spodumene锂辉石: two, perfect (note extremely flat, smooth breaks), fluorite: four, perfect]

Food for thought: Far from being a matter limited to academic interest, knowledge of gem cleavage has practical value, both as a means of gem identification, and in the appropriate fashioning and selection of gems for a particular use. (Answers to the questions below are found at the end of the lesson).

Question 1: Suppose you're a budding gem cutter or collector, and you happen to be at a swapmeet where a vendor has some transparent pink gem rough to sell. He knows that it is either Kunzite (pink spodumene) or pink tourmaline, but just can't remember which one. You have been wanting some pink tourmaline, so you look at the material closely and can't find any evidence of cleavages, even using your 10 power magnifier. Of the two choices, which is it most likely to be?

Question 2: A big decision is coming up in your life--> you are about to choose an engagement ring. Not being a slave to tradition, you are considering a colored stone for the piece, rather than a diamond. You want a blue stone, and your top contenders are: blue topaz, and blue sapphire. Considering that engagement rings are worn all day, every day, for many years, you do not want a stone that is likely suffer a cleaveage that will crack or break it. Which is your best choice? (Hint: look up topaz in the Lyman text pg. 128).

Question 3: You've found a beautiful piece of apatite rough and want to have a stone cut from it . You approach your friend who is a facetor, and ask him/her to cut you a marquis shaped stone from the piece. The cutter declines and says they will cut an oval or round but not a marquis. Why?

Miners have long used the cleavage properties of gems in trimming the stones they find. "Cobbing" is the act of smacking a piece of rough sharply and precisely with a hammer to break off any unstable (already partially cleaved), or included areas. Knowledge of the cleavage planes in the material being mined is essential to efficient use of this technique.

The use of cleavage is perhaps most well known in diamond cutting. We've all seen photos or videos of that tense moment when the diamond cutter inserts the wedge at a particular spot on the diamond and strikes it with a mallet. If all goes well, the stone splits precisely where the cutter wanted, and expected, it to. It is said that the expert that first cleaved the (up to that time) largest rough diamond ever found (The Cullinan) had studied it for months to determine its cleavage planes, and upon striking the first blow fainted dead away from anxiety. All was well, however.

**Check the text: See page 7 in the Hall text to view the largest of the many cut stones from the Cullinan, in its home in the Royal Scepter of the British Crown Jewels).

Fracture: Whereas cleavages occur only in some gems, and within those, only in certain directions, fractures can, and do, occur in all gems, and in any direction. A fracture is a break which is not along a cleavage plane. With sufficient force, any gem will fracture, although some do so more readily than others. The edges of fractures are not smooth like those of cleavages, but they do tend to have one of several basic appearances.

Playing on the resemblances of certain fracture types to well known surfaces and objects, terms like conchoidal (shell-like), splintery, uneven, step-like, and granular are used. Like cleavage, this is a species specific characteristic which has value in the identification of gems.

[Citrine quartz: conchoidal, Charoite: splintery]