It is interesting to note that several of the synthetic gemstone processes are based on wartime efforts unrelated to gemstones. Such as the case with synthetic diamonds.
Synthetic diamonds were needed during World War II, due to the lack of availability of natural diamonds for war time production. And in fact, the recently resolved issues revolving around DeBeers not being able to come into the United States due to an outstanding legal problem, was due to DeBeers trying to control the production of industry grade synthetic diamonds from the time of that war.
The production of hydrothermal quartz is another process with seeds in the World War II efforts. Quartz crystals were necessary for production of radio and other electronics during the war. But only high quality crystals from Brazil were of use for the high tech production. And since the routes to the Brazilian fields were blocked to German scientists, it was the Germans who perfected the process during the war, to take the place of the unavailable natural stones.
The Process
The process itself is rather simple, although the actual production involves a great deal of finely tuned science to make it work properly. In general, an autoclave is set up with a high and low end as shown in the graphic you will see in just a few minutes.
The autoclave is lined with gold, and has heaters running the length of the unit. The heaters are tuned so that the bottom is much hotter than the top line of heaters. Thereby making a significant heat differential in the internal temperature of the autoclave.
Next, a solution of nutrient rich material is added, with the concentration of material going to the bottom. This material is usually low quality milky quartz that is of abundance but very low quality. This is heated in the lower section until it become hot enough to liquidate, and to rise to the top of the autoclave. There the cooler temperature causes the molten quartz to cool enough that the quartz comes out of its liquid state and attaches itself to a seed crystal of high quality quartz, natural or synthetic, that allows the seed crystal to grow. Once the liquid has cooled, it falls back down to the bottom of the autoclave by a natural convection current established by the variable heat and gravity. There it is heated and again becomes enriched with nutrients and starts the process again.
Let's take a look at the graphic of this autoclave and see how it works.
The result is quartz crystals that can be grown quite large and of very high quality. This production allowed for the vast majority of quartz used in electronics including the quartz crystals used to make quartz watches. And by adding certain elements to the mixture, synthetic amethyst can be produced. Below are two photographs of a large synthetic amethyst grown by the hydrothermal process. Notice in the photograph on the left that the crystal is of uniform color and clarity. However, when you turn it the right direction, the seed crystal used to start the process is visible. This piece is approximately 6 inches in height.