What is Hydrothermal Quartz?

I stumbled on a shop selling beautiful little pendants a few days ago with very pretty looking stones, called "hydrothermal quartz". So let's see what that means.

Quartz is one of the most common minerals in nature, and the price of natural quartz is quite low. Only the special colored varieties, such as violet amethyst and yellow citrine, command prices over a few dollars a carat.

Given the low price for natural quartz varieties, it may come as a surprise that there is a large industry producing synthetic quartz using a method known as hydrothermal transport and recrystallization.

Synthetic Hydrothermal Ametrine QuartzSynthetic Hydrothermal Ametrine Quartz
Though the cost of producing synthetic quartz is not much less than mining natural quartz, a large industry has developed to create synthetic quartz for modern technology. Massive quantities of quartz are used as a piezoelectric material in oscillators, gauges, microphones, clocks and watches.

The hydrothermal transport method uses a large autoclave; an electrically heated, pressure-sealed container 3 meters or more in height. The autoclave contains an aqueous solution of sodium carbonate or hydroxide. Small fragments of quartz that act as a source material are placed on the bottom. The upper portion of the container is a cage that supports numerous small seed crystals of quartz. When the temperature is raised to about 400 degrees centigrade, the quartz fragments dissolve and crystallize onto the seeds in the cooler upper portion of the container.

So basically, it is glass. It's not a gemstone at all.  No matter what it's called, whenever the word "hydroquartz" is found, it's GLASS.