Toughness may be defined as the resistance a gemstone offers to breakage. In practice this is considered to include resistance to chipping and cracking as well as actual breakage. If a gem has low degree of toughness, it is impractical for many uses and its value is reduced accordingly. A gemstone that breaks very readily is said to be FRAGILE.
A stone may be very tough and not extremely hard, or vice versa. But a mineral is not suitable for use as a gemstone if it separates so easily that it is almost entirely lacking in toughness. A diamond is extremely hard but not exceptionally tough. Fluorite is neither hard nor tough and is therefore fragile. Nephrite is not hard but it is extremely tough. Stones that are very tough may wear better than much harder stones. The qualities of hardness and toughness must not be confused.
All substances are considered to be composed of small particles called ATOMS. These minute particles are held together by a force of attraction called COHESION, which tends to resist any separation between the atoms. When a substance is broken it is done by a force that over comes this attraction. Thus the stronger the cohesion, the greater will be the effort necessary to break the substance.
ELASTICITY is the ability of minerals to undergo a certain amount of distortion when under pressure and yet return to the original form when the pressure is released. Some minerals (metals) may be cut (SECTILE), hammered flat (MALLEABLE or drawn into wire (DUCTILE).
BRITTLENESS refers to a lack of ability to withstand distortion without separation. Minerals that break or powder, instead of bending or flattening out when struck with sufficient force, are said to be BRITTLE. Thus brittle does not necessarily mean easily broken; it only means that a mineral finally powders rather than flattening as a metal does under enough blows. All important gem minerals are classed as brittle minerals, which are subdivided as follows: FRIABLE (easily separated into grains, SOFT (easily powdered) and TOUCH (offering some degree of resistance to separation). Toughness in gemstones is classed as EXCEPTIONAL, EXCELLENT, GOOD, FAIR OR POOR.
The distinguishing feature of crystalline materials is the regularity of the arrangement of atoms. In many gemstones the pattern is such that the spacing of atoms in different directions is substantially the same. When such a stone is broken, the break is likely to take whatever path will relieve the stress that caused the break. In other gemstones the arrangement of atoms is such that some planes have concentrations of atoms, with wider spaces between these planes. In this event, a break in a stone is likely to occur parallel (i.e. between) the planes of concentrated atoms (Figure 1). Depending on the symmetry in the crystal system and the directions in which atoms are more closely packed, there may be six, four, three or two directions of cleavage or only one direction. The cleavage of a mineral is classed as PERFECT, DIFFICULT, EASY, etc.
A more concise definition of cleavage and its characteristics would be: Cleavage is the tendency of a crystalline mineral to separate in certain definite directions producing flat, smooth surfaces. The planes in which it occurs are called CLEAVAGE PLANES. Since these correspond to definite atomic planes, they will be parallel to a possible crystal face of a crystal form in which that mineral might occur. For example, diamond occurs in several different forms characteristic of the cubic system; however, the atomic planes in which an easy separation may be effected are parallel to the faces of the octahedron form. Thus diamond is said to have OCTAHEDRAL CLEAVAGE (Figure 2). (Note: In the stone samples that accompany this course, notice the excellent three directional cleavage in calcite.) The cleavage planes in diamond are often referred to as the GRAIN.
A separation involving cleavage does not always produce absolutely Smooth surfaces, but may exhibit tiny "ledges" or a step like appearance, resulting from a combination of alternate cleavages and fracturcs or cleavages in different directions. Some minerals leave easily in directions that parallel the faces of more than one crystal form.
FALSE CLEAVAGE, or PARTING is a separation somewhat similar to cleavage. It frequently occurs along planes between laminae due to repeated twinning, thus differing from true cleavage, which occurs between atomic planes. Parting often occurs only in certain specimens of a gem species. Among important gem minerals, it occurs only in corundum, where it is seldom sufficiently pronounced to affect toughness or to either assist or interfere with fashioning.
When a cutter knows the direction of the cleavage plane and the "cleavage is good", a slight cutting, followed with a wedging strain, will accomplish the splitting. Cleavage was once important in diamond fashioning, but it has been largely replaced by sawing. Flawed portions of a diamond crystal can often be cleaved off with very little loss of material.
Cleavage is so well developed in some gemstones that they must be handled with extreme care. A sharp blow or rapid changes in temperature, such as might occur during cutting and polishing, may start cleavage cracks and lessen the beauty and value of a gem. Also, once the slightest cleavage is apparent, the flaw may increase with hard usage.
The term FRACTURE is used to describe the chipping or breaking of a substance in any direction other than that of a cleavage plane (Figure 3).
A CONCHOIDAL (pronounced kon-KOY-dal), or SHELL LIKE, fracture is the term used to describe breakage that produces curved ridges similar to the outside markings on a shell. A typical conchoidal fracture occurs when, glass and most transparent colored stones are chipped.
GRANULAR, SPLINTERY, UNEVEN AND EVEN are terms used to express the appearance of other types of fracture. A granular fracture is typical of crystalline aggregates; for example, the broken surface of a lump of sugar. Splintery is used to describe a break with a fibrous appearance. An even fracture approaches the smoothness of a cleavage. An uneven fracture includes any break not covered in the above descriptions and may be irregular, jagged, etc.
It has been customary in the trade to refer only to breakage on the girdle or on facet junctions as fractures. However, fractures may be on the surface of a gem, where they are sometimes known as PITS, or in the interior, where they are often called FLAWS or FEATHERS. The term CHIP is often used to describe surface fractures, especially on the girdle.
Flaws such as fractures and cleavages spoil the transparency of a stone and lower its value. When they are observed, special care should be taken when setting or otherwise subjecting the gem in which they occur to possible blows or pressure. Another kind of imperfection that cart increase the possibility of separation is INTERNAL STRAIN. This can be the result of improper crystallization or inclusions that tend to distort the structure of the gem. The detection of internal strain will be discussed under the subject of optical properties.
THE EFFECTS OF CLEAVAGE AND FRACTURE
A few stones cleave so easily that their desirability as gems is affected. For instance, genuine topaz (not topaz quartz), when set in rings that receive exceptionally rough usage, may prove unsatisfactory because it cleaves easily. Many varieties of zircon fracture easily, and even in ordinary wear as ring stones they crumble or fracture along the edges of the facets or even on the facet surfaces themselves.
Some gem minerals fracture easily but do not cleave easily, for instance, zircon or emerald. In others, cleavage is more likely to occur than fracture, topaz or diamond, for example. However, if one is familiar with their characteristics and exercises care in their wear, there is but slight chance of serious damage to any gemstone. It is the responsibility of the salesman from whom a gem is purchased to acquaint the purchaser with any of the ordinary properties in which it is in any way deficient.
HOW CLEAVAGE AND FRACTURE MAY BE USED
Cleavage is important to both the lapidary and the diamond cutter, but for different reasons. Because of the extreme hardness of diamond, shaping the rough before faceting is difficult. Cleavage provides a means of separating large diamond crystals into better shapes or sizes. The lapidary, who works with softer gemstones, finds cleavage something to guard against rather than to use. Some gemstones (kunzite, for example) have such easy cleavage that it is exceedingly difficult to cut.
Experienced gemologists can often obtain an indication of the identity of a stone by the directions and nature of a cleavage crack or by the shape and luster of fractures, but considerable handling of many and various stones is necessary for one who wishes to depend on these as an aid in identification. Of course, such a test is limited because the majority of stones do not cleave easily nor display characteristic fracture surfaces. In fact, most gemstones display a conchoidal fracture with a VITREOUS (glasslike) luster. The important exceptions are listed in the table accompanying this assignment, and include such examples as turquoise and its best imitation, glass. Both turquoise and glass exhibit a conchoidal fracture, but the luster of the fracture surface of glass is vitreous and that of turquoise dull or earthy, similar to that of unglazed porcelain. Since almost any cabochon will have minute fractures around the girdle, this becomes a valuable test. Glass is used also to imitate "black onyx" and other varieties of chalcedony (quartz). Although chalcedony, like glass, displays a conchoidal fracture, the luster of the fracture surfaces of chalcedony are waxy rather than vitreous.
Cleavage is less valuable in identification, but, like fracture, the presence of tiny cleavages can sometimes be of assistance. For example, some varieties of chalcedony (e.g. moonstone) bear a close resemblance to certain types of feldspar. Any chips or breaks along the girdle of the precious moonstone (feldspar) will display tiny flat cleavage surfaces. These may or may not be observed easily, depending on the general direction of the break. If the break closely parallels to the cleavage planes, the exposed surface will be flat, if it crosses the cleavage planes at an angle, the exposed surface will have a step like appearance, resulting from a combination of alternating cleavages and fractures.