Synthetic opals are man-made opals that have the same chemical composition, internal structure, physical properties, and appearance as natural opals. They are often called lab-created opals, lab-grown opals, or cultured opals to indicate their man-made origin. Synthetic opals can exhibit a spectacular play-of-color appearance that often exceeds the beauty of many natural precious opals. They are produced in a wide range of colors and patterns that many people enjoy. Many synthetic opals look so much like natural opal that trained gemologists can have difficulty separating them from natural opals. This is why whenever synthetic opals are advertised or presented for sale, sellers are required by law to clearly communicate that they are manufactured by people and they are not natural opals.
The cause of opal’s play-of-color was discovered in 1964 using an electron microscope. An array of tiny silica spheres, with a uniform size of less than ½ micron in diameter, in a close packing arrangement, serves as a natural diffraction grating to separate visible light into the colors of the spectrum.   This discovery revealed how opal’s fabulous play-of-color is produced and provided the blueprint for making a synthetic precious opal.
The first synthetic opals were made by precipitating tiny silica spheres of uniform size, and allowing them to settle into a close packing arrangement. The spaces between the spheres were then filled with a binding medium that would harden, hold the structure together, and allow the diffraction of light. 
Creating synthetic opal was different from the creation process for most other synthetic gem materials. Other gem materials are single crystals, and growing the crystals is the key to producing the gem material. Synthetic opal creation presented multiple challenges: creating millions of spheres of identical size; settling them into perfect arrays (which requires as much as a year or more of time); and, binding the spheres together into a material with a durability that is suitable for a gem. Binding the spheres often requires impregnation of the opal by a polymer resin, an ingredient that is not in natural opal. In addition to improving durability, the polymer resin can improve translucence, luster, and color. Synthetic gem materials are often treated for the same reasons as natural gem materials.
The main reason that people produce synthetic opal is a hope of being able to manufacture it at scale and at a price that is lower than natural opal. They have been incredibly successful. Many types of synthetic opal are now cut into beautiful ring-size cabochons that sell for just a few dollars each. Even the very best synthetic opal can be cut into cabochons and sold for just a small fraction of the price of natural opal cabochons of similar size and appearance. Synthetic opal certainly wins some buyers away from natural opal, but it is not likely to displace natural opal from the gem and jewelry market. Why? Most people who love opal are glad to pay a higher price to own a gemstone that formed within the Earth - and, in their opinion, no synthetic material will ever compete with that! These people insist on the real thing!