Ask A Scientist

What makes a ruby red and how do you decide the clarity of a diamond?

Asked by: Emily Puthawala
School: St. James Middle School
Grade: 7
Teacher: Mrs. Walter
Hobbies/Interests: Horseback riding and tennis
Career Interest: Horse trainer

Answer from Daniel Brennan

Adjunct Lecturer, Binghamton University

PhD school:
Binghamton University

Research area:
Solid State Chemistry, Chemical Education.

Wife, Alison, who teaches chemistry at Chenango Forks High School.

Movies and Travel.

The questions that you have asked require that we consider both what makes up a substance and how it forms. What we call a ruby is almost entirely composed of aluminum oxide in its corundum form. Corundum has a crystal structure made up of repeating units of aluminum ions surrounded by oxide ions. When only aluminum and oxygen are present, visible light does not interact with the crystal to give the gemstone a color and it is therefore colorless. When certain impurities are present, these can interact with visible light to cause the gemstones to display different colors. The impurities, which are typically metal ions with a size and charge similar to aluminum, can replace aluminum ions in the corundum crystal. This replacement of aluminum with another metal is an imperfection, or defect, known as substitution. The impurity that is most important for producing the red color in rubies is chromium. The chromium is present in very small amounts, typically replacing only around 1% of the aluminum. Despite such a small number of substitutions at the submicroscopic level of atoms, the result is dramatic and clearly visible to our naked eyes as a brilliant red gemstone. In fact, ruby is a specific name given to red corundum. If the stone is any color other than red, including the colorless case where no substitution occurred, the crystal is known as a sapphire. Without specific guidelines and because both are based on corundum, there is no simple way to distinguish a dark pink sapphire from a light red ruby. Your second question about diamonds also depends on the presence of crystal defects. Diamonds, which are a crystalline form of the element carbon, can likewise undergo atomic substitution with small amounts of impurities to yield colored stones, such as blue diamonds when boron is the main impurity or yellow diamonds when nitrogen is the main impurity. While such substitutions are considered when determining a diamond's color, these defects occur at the submicroscopic level and do not cause the stone to appear cloudy. Instead, the defects that are important for grading a diamond's clarity can be seen under a microscope or, if large enough, even with the naked eye. During the formation of a diamond, it is possible that small crystals of another material or even small crystals of other diamonds can become trapped in this larger diamond. These small crystals are now 'included' in the diamond and are therefore known as inclusions. The overwhelming majority (roughly 80%) of diamonds recovered through mining have such extensive inclusions that they are not suitable for use as gemstones. Of the remaining 20%, most diamonds have at least some observable inclusions. A diamond's clarity is determined by the size and number of inclusions visible in the sample when viewed under 10x magnification. As a rule, diamonds with no visible inclusions are the most valuable because they are so rarely found in nature. Scientists have developed such a deep understanding of the processes involved in diamond formation that synthetic diamonds can be made with essentially no inclusions. To learn more about scientific topics such as these stop by the annual Southern Tier Science Olympiad held on the Binghamton University campus this Saturday. Hundreds of area high school students, representing nearly 14 schools, will face off in a daylong series of contests designed to test their knowledge in diverse fields such as chemistry, cell biology, structural engineering, astrophysics and mathematics. There will even be a specific event on Rocks and Minerals, which makes your questions especially relevant for this week's column.

Last Updated: 9/18/13