Rainbow Crystals have a pattern of colors like a rainbow in the sky     Index to Quartz Digging Cleaning Worth Fee Pay Mines Types Forms Inclusions Geology Mineralogy Metaphysics Synthetic Gemstones Handedness Experiments Photos from readers

Rainbow Quartz Chapter 1   Page 6 b OH, THOSE RAINBOWS! I have seen many internally fractured quartz crystals during my collecting years, but it wasn't until just a few years ago that I heard someone making a fuss over "rainbow" quartz. At the time, I wondered what the hoopla was all about. It's just a quartz crystal with an internal fracture, either of natural origin or induced by the careless practice of removing the crystal too quickly from a hot acid bath, right?      But since then I have had some interesting conversations with various collectors and learned some additional facts. Ken Silvy, who digs herkimer diamonds (double terminated quartz crystals) several months a year up in the Middletown, New York area, told me about how he has to handle the crystals when he takes them out of the ground. He keeps them in the cold clay and places them in a bucket of cool water in the shade. He says he has seen many first-time collectors ruin crystals by wiping off the clay and putting them in the sun to dry. POP! CRACKLE!! The crystals heat up rapidly on the outside while the inside is still cold. Thermal stress builds up until they fracture! They must warm up very slowly to prevent the thermal stress building to the point of overcoming the internal strength of the crystal.      Some time later, Ron Coleman told me an interesting and somewhat sad story relating to a specimen from a tremendously large pocket of crystals he and his brother Jimmie dug near Jessieville in Garland County, Arkansas. Some of these crystals were 3 feet in length and 1 foot in diameter. One in particular was clear from the termination to over one-half its length! Within a minute after it was removed from the cold ground, a fracture appeared about a foot from the base. They quickly covered it with blankets, but it was too late. While they watched in amazement (and disappointment as they saw $ 's flying out the window!), the fracture grew up the length of the crystal to within 6 inches of the termination. To prove his point, Ron took me over to Jimmies' backyard, pulled back some blankets and showed me the specimen!      After these conversations, I became seriously interested in finding some specimens of "rainbow" quartz for further examination. Most crystals that I have seen that show rainbows were interesting but not all that attractive. But then, while on a trip visiting quartz mines, shops, and miners with my wife over the Labor Day 1999 weekend, I came across the one specimen exhibiting this feature that I knew I had to buy. It was at Sonny Stanley's shop on a table with many other specimens of natural light smoky quartz. Sonny told me he had recently purchased all this smoky quartz from Danny Rickard, who had recently dug it from a site near Paron, Arkansas. After careful examination, I discovered that most of the specimens were badly damaged during digging, but I managed to find 11 pounds of crystals that still had the terminations intact. I bought all that I thought were any good for $100. In this batch was the one special crystal I saw that displays a spectacular rainbow. See picture.      The specimen weighs about one-half pound and has very little internal fracturing except for the one major fracture that is about one-half inch deep from one of the terminal major rhombohedral faces. The fracture parallels the rhombohedral face and forms an oval that measures about five-eighths inch by one-half inch. See picture.      I took pictures of the rainbow area with my digital camera and when looking at the close-up picture of the reflecting fracture was struck that I had seen this color pattern before - many times. I got out my old college optical mineralogy book and opened it up to the Interference Color Chart. There it was - staring me right in the face - the explanation   of the color patterns of the rainbow crystal. I thought for awhile, but could not remember reading a scientific explanation of this natural phenomenon. So I decided I must write it down. The Explanation For years I had seen what happened when a single drop of oil landed on a wet concrete driveway or street. A rainbow of color appeared. I had recognized that this rainbow reflection of light was directly related to the thickness of the film of the oil floating on the water. I could see the orderly pattern of 1st order gray, then yellow, then orange, red, violet and blue starting from the thinnest edge of the oil film and progressing across thicker zones of the film. Then 2nd order colors appeared where the oil coating was thicker - blue-green, green, yellow, orange, red, blue. The pattern of colors was always the same, but the colors became more pastel with each rise of color order.      I also have seen many examples of natural lightly iron-stained quartz that display iridescence on the iron coating. This is due to refraction of light through a very thin surface film of iron oxide. General color bands of electric yellow, blue, green, and red are sometimes seen, but are irregular, thus generally preventing a pattern developed from gradual thickening or thinning of the layer. Rarely, a mineral specimen will be seen that has a very uniform iridescent color, such as all electric blue, green, yellow, or red. I have seen some examples of this occurring on specimens of Arkansas goethite from Polk County. Some examples of rainbow films on goethite, listed as turgite in old mineralogical texts, are know from a site in Garland County, Arkansas.      The same reflection explanation holds true for rainbow quartz, but the light interaction does not depend on this thickness of a coating of one substance on another, but instead it depends on the space between the walls of the fracture inside the crystal. The fracture contains a vacuum since it does not reach the crystal's surface at any point. In the photographed specimen, there are distinctive bands of 1st order colors on the outer band of the fracture and 2nd order colors into the center of the fracture. The center of the fracture, in this case, was the initiation point of the fracture during stress release. The fracture radiated out into the crystal in a planar direction until the stress was released, and the fracture died out before reaching the crystal's surface. I do not know what caused the specific location of the stress release point, but it could have been a tiny flaw, like an inclusion, in the specimen.      The fracture developed on a rhombohedral crystal plane of the specimen. This plane is a known cleavage direction in quartz, but not often seen. The difference in colors across the fracture is caused by the thickness of the gap between the fracture walls. From looking at the Interference Color Chart, it appears that the widest portion of the gap (occupied by a vacuum in this case and located in the center of the oval rainbow) is a little greater than 0.12 mm, taking the birefringence of quartz as 0.009 from the chart. It runs out to 0.01 near the edge of the fracture before the fracture displays no color and where the fracture dies out.      Maybe it looks magical, but a little knowledge of optics, optical mineralogy, physics, and optical phenomena are all that is necessary to explain this "mystical" property. By the way, this specimen is not  for sale or trade since it is the best and most symmetrical display of a rainbow I have ever seen inside a quartz crystal! Ch1. Page 6 Contact the authors of Rockhounding Arkansas Revised Sept 1999 © Rockhounding Arkansas 1998 http://rockhoundingAR.com