Structure of Color Gemstones

The competent gemologist has less need for a detailed knowledge of the chemical properties of gemstones than he does for optical and other physical properties. Nevertheless, there are times when a fundamental knowledge of the structure of matter and of basic chemistry is of great value. Minerals are classified by the mineralogist on the basis of chemical composition. The manner in which the atoms that compose them are joined together is a major factor in determining physical properties, and the nature of the atoms has a major bearing on chemical properties. In the next few pages a brief, fundamental background for chemistry is given.

This is included to provide basic information for those who would like a better understanding of the fine structure of gem materials, the causes of color in transparent gemstones, and the nature of inclusions. Because of the nature of the subject and the fact that an understanding of it is not required for a mastery of grading and identification, no questions will be asked about the structure of matter on examinations.

Year by year, during the last half century, the study of the fine structure of matter has intensified and the value of that knowledge has become increasingly obvious. Many years ago it was shown that elements were related in certain definite ways. It became clear that each step upward in atomic weight was a step upward in complexity of structure. Niels Bohr, the Danish physicist, demonstrated that atoms could be visualized readily as minute solar systems, with massive, positively charged particles (PROTONS) in the center and with less massive, negatively charged particles (ELECTRONS) moving rapidly in orbits about it. Each step upward from element # 1 to element # 92 means an increase of one proton in the nucleus of an atom and one more electron moving rapidly about it. The nucleus of atom is composed of the densest substance known to man. It consists of positively charged protons and uncharged particles called NEUTRONS. The density of neutrons and protons is so great that, if it were possible to pack the nuclei of atoms together until they reached the size of a child's marble, the weight of the sphere would be approximately 250 million tons ! The negatively charged electrons have a density of roughly 1/1800th of a proton, although their size is comparable.

The miniature solar systems increase in complexity in steps as more protons, more neutrons and more electrons are added with increasing atomic number. Thus hydrogen, which has one proton and one electron, finds one electron moving in all directions about its single proton nucleus. In helium, the element with the atomic number of two, there are two electrons that move about it the same distance from the center. The next eight elements have atoms in which the additional electrons move in a second energy level outside of the first level, in which only two are located. The next step upward is another energy level figure that has from one to eight electrons, depending on the atomic number of the element, and this is followed by a level in which eighteen may be found and another with a maximum potential of thirty two. The last energy level is known to be an incomplete one in which of the natural elements found on each, uranium, #92, is the most complex. It has six electrons in its outer shell, with a maximum number in each of the inner shells. The artificial radioactive elements made in the particle accelerators of nuclear physicists are made by adding protons to the center and additional electrons to the outer ring.

The manner in which an element behaves chemically is determined in a large measure by the relationship of the number of electrons in its outer ring of electrons and the number that would be necessary to complete that energy level. Since the innermost energy level contains two electrons, helium, which has only two, has a complete first energy level. It is inert chemically. There is no tendency for it to combine with other elements; thus it is called a NOBLE GAS. In steps of increasing complexity, up through the periodic table of elements in each situation where the outer ring has a full complement of electrons, the element is a noble gas. In general, those elements that lack one, two or three electrons of having full outer rings are the nonmetallic elements, including the halogens (fluorine, chlorine, bromine, etc.), oxygen, sulphur, selenium, tellurium, nitrogen and phosphorus. Generally, those that have only one, two or three electrons in the outer ring are metals.

Compounds are formed in various ways. The most important is that in which electrons are transferred from a metallic element with one, two or three it can "spare" to a nonmetallic element that is lacking electrons in its outer energy level. Table salt, for example, is formed by the combination of sodium and chlorine; sodium has only one electron in its outer ring and chlorine lacks one. In effect, sodium lends an electron to chlorine and the combination forms a new material with properties unlike those of either of its constituents. If a material such as chlorine (Cl) which lacks one electron in its outer ring, combines with a material such as carbon (C), which has four, it requires four chlorine atoms to provide four spaces for the extra electrons of carbon. Under these circumstances, chlorine is said to have a VALENCE of minus one and carbon is said to have a valence of plus four (+4), meaning that it has four to contribute from its outer ring. Thus a compound of these two elements is abbreviated as CCl4.

Under certain circumstances, elements behave differently. For example, chlorine could give up seven electrons instead of taking one. Iron (Fe) may have a valence of +2 or +3; therefore, it may form compound with oxygen (valence minus 2) , either as FeO or Fe2O3. Since oxygen has a -2 valence, FeO is Fe+2O-2 and Fe2O3 must be Fe2+3O3-2. Fe2O3 is the formula for hematite. Ordinarily, a compound must have a balance such that positive and negative valences add up to zero; thus spinel is MgAl2O4. Magnesium (Mg) has a valence of +2 and oxygen (O) is usually -2, so aluminum (Al) must be +3, because MgAl2O4 would be +2, ?,-8. 2Al must be +6, so Al = +3. In each compound in which there are metallic and nonmetallic elements, positive and negative valences are balanced. Some compounds containing only nonmetallic elements do not pass an electric current; therefore, the idea of positive and negative valences does not apply. These are called COVALENT, since they share electrons rather than transfer them one to another.

Because the most abundant and gemologicaly most important elements have low atomic numbers, they have relatively simple atomic nuclei. It is interesting, too, that elements of even atomic number are much more abundant than the odd numbers.