Opals

The Harvard University opal collection is a modest sized assemblage of 280 hand-sized specimens that have been bequested and donated to the university throughout the past 200 years.  Opal, while not truly a mineral in the strictest sense of the definition, is still maintained as a part of the MGMH collection due to its mineral-like nature, the history of the opal as a gemstone, and for its value in educating the public about the beauty and fascinating qualities of precious earth materials.

Despite enjoying thousands of years a well-loved gemstone, opals are not truly minerals. A true mineral must naturally occurring, not biological in origin, stable at room temperature, and have an ordered atomic arrangement with a regular chemical formula. Opals follow all rules but the last- they are considered mineraloids due to the fact that they are composed of microscopic spheres of silica in irregular arrangements.

Opal’s main component is SiO2 and H2O formed as a precipitated silica colloid from volcanic or sedimentary silica rich rocks. Other chemical element such as Fe, Al, Na, K, Ba, etc. are present as trace elements either on the precipitating fluids or from mineral inclusions. The SiO2 particles are suspended and cemented by a non-ordered smaller particle medium, close-packed amorphously or near amorphously.

The variety of colors and the internal 'fire' seen in precious opals is due to the optical effect known as iridescence.

The iridescence seen in opals is due to tiny, evenly sized spheres of silica ordered in such as way as to cause a combination of interference and diffraction of light through the specimen.The size of the silica spheres impacts the colors seen. The visible colors are determined by the spacing between the planes of silica spheres and the orientation of planes with respect to the source of light. As the angle of the stone to the light changes and the orientation shifts, we see dramatic changes in color that make opals so special.

Opals continue to be a source of fascination for gemologists, scientists and the public. Research into the color play, structure and development of new synthetic opals continues to illuminate and expand our understanding of these precious stones.