Synthesis 101: The Hydrothermal Method

Gemmology is the study of gems, precious and semi-precious minerals used mainly in jewelry. Among the fascinating subjects of this science, gem synthesis occupies a special place (I love it!). Gemstone synthesis enables the laboratory creation of gems with physical, chemical and optical properties similar to those of natural stones.

One of the most widely used methods for gem synthesis is the hydrothermal method. This is where you’ll find the vast majority of synthetic quartz, as well as many beryls, including emerald. It is also used for corundum, but is not the same as the Verneuil method.

I’ll try to explain it as best I can!

What is the Hydrothermal Method?

The hydrothermal method is a process for growing crystals in an aqueous solution at high temperature and pressure. It is inspired by the natural conditions under which certain minerals form deep in the Earth, where hydrothermal solutions (hot water loaded with dissolved minerals) deposit crystals as they cool. This is the main natural formation process for quartz, for example.

History and Development

In the 19th century, hydrothermal technology focused on the synthesis of minerals such as quartz crystals, with innovations such as steel autoclaves and gold liners, enabling the synthesis of over 80 mineral species. European countries were at the forefront of this research.

In the early 20th century, centers such as the Carnegie Institute’s Geophysical Laboratory in the USA, with researchers like Bowen, Morey and Niggli, made significant advances in the study of phase equilibria and water solubility.

The 1950s saw the introduction of versatile devices and Japanese domination of quartz crystal growth, while the Soviet Union focused on the synthesis of new compounds.

In the 1970s, despite the challenges of growing large crystals, hydrothermal research explored new methods and diversified, with Japan leading the way in modern research, contributing to varied applications in contemporary science.

Today, the hydrothermal method is still at the heart of research beyond the world of crystals. it is still used to synthesize some synthetic beryls, in particular emerald, some corundum and all quartz (with the exception of polycrystalline quartz such as chalcedony).

The Hydrothermal Process in Detail

The hydrothermal method is a fascinating technique used to create crystals in the laboratory, mimicking the natural conditions under which gems are formed. Let’s explore this step-by-step method, with details adapted to different types of crystals such as quartz, emerald and corundum (ruby and sapphire).

Solution Preparation

The first step in the hydrothermal method is to prepare a mineral-rich solution. For quartz, for example, a silica-saturated solution (SiO?) is used, while for emerald, the beryl solution must contain silicon, beryllium, and traces of chromium or vanadium. Corundum (ruby and sapphire), on the other hand, requires a solution of alumina (Al?O?) with additives such as chromium for rubies and iron or titanium for sapphires. Alkaline substances such as NaOH or KOH are added to these solutions to increase mineral solubility.

The Autoclave

Once the solution has been prepared, it is placed in an autoclave. An autoclave is a steel vessel capable of withstanding high temperatures and pressures. It is divided into two zones: the dissolution zone, at the bottom, where the temperature is higher to dissolve the source material, and the growth zone, at the top, where the temperature is slightly lower to allow crystal precipitation and growth.

Nutrients and Sprouts

Nutrients and germs must be prepared for the crystals to form. The source material, such as ground natural crystals, is placed in the dissolving zone. Small crystals, called sprouts, are placed in the growth zone. These seeds serve as the basis for the growth of new crystals.

Growth Process

The heart of the hydrothermal method is crystal growth, which takes place over several weeks or even months. At high temperatures, the minerals dissolve in the alkaline solution in the dissolution zone. The mineral-enriched solution rises towards the growth zone, where the lower temperature favors the precipitation of dissolved minerals onto the sprouts. Crystal growth can take from several days to several months, depending on the desired size and quality. Each type of crystal requires specific temperature and pressure conditions.

Energy Consumption and Sustainability

The hydrothermal method requires rigorous resource management, as the process is energy-intensive. Autoclaves consume a lot of electricity to maintain stable conditions of high temperature and pressure. Technological innovations aim to improve the energy efficiency of autoclaves, thus reducing costs and environmental footprint.

Quality Control

To guarantee high-quality crystals, strict quality control is essential. Controlled conditions minimize inclusions and defects in the crystals. This method produces crystals of great uniformity, often superior to that of natural stones.

Specific Differences According to Crystal Type

Each type of crystal has its own specificities in terms of synthesis conditions and application.

Quartz

• Application: Used in jewelry and electronics for its piezoelectric properties.
• Conditions: Temperature from 300 to 400°C, pressure from 1,000 to 3,000 bar.
• Duration: 1 to 3 months.

Emerald

• Application: Prized in jewelry for its bright color and clarity.
• Conditions: Temperature from 400 to 600°C, pressure from 700 to 1,500 bar.
• Duration: 2 to 6 months.

Corundum (Ruby and Sapphire)

Duration: 1 to 4 months.

Application: Used in jewelry and industrial applications such as lasers.

Conditions: Temperature from 500 to 700°C, pressure from 1,000 to 1,500 bar.

Advantages and limitations of the Hydrothermal Method

Benefits

• Quality: Synthesized crystals are often of very high quality, with few inclusions and defects.
• Size: Produces large crystals.
• Variety: Allows synthesis of various gems with specific properties.

Limitations

• Cost: The process is costly due to the equipment required and energy consumption.
• Time: Crystal growth can be slow.
• Specific conditions: Precise temperature and pressure conditions must be strictly controlled.

Applications in Jewellery and Industry

Hydrothermal gems are commonly used in jewelry to create affordable jewels while maintaining high aesthetic quality. They also have industrial applications, notably in electronics, where synthetic quartz is used for its piezoelectric properties.

The evolution of synthetic methods has also enabled consumers and designers to do without natural gems, which are non-renewable and often at the heart of social and environmental controversies.

Recognizing Synthesis: Emerald, Corundum and Quartz

Identifying synthetic gems is important to differentiate between real and fake stones. Here’s how to distinguish between emeralds, rubies and sapphires, as well as synthetic quartz, and what they represent on the market.

Recognizing Synthetic Beryls: Emeralds, Aquamarine, Morganite, Heliodore, etc.

• Natural beryls often have distinctive inclusions and emeralds are rarely pure, while synthetics have different inclusions (nailhead or zig-zag) and will mostly be very pure.
• Natural beryls vary in color, while synthetics can have a uniform, clean, saturated hue.
• Synthetic beryls are common on the market, especially for emerald. They are often used as an alternative to mining practices and for purer, more affordable stones.

Recognizing Synthetic Corundum: Sapphires and Rubies

• Synthetically produced corundum is generally very pure. they can show very geometric striations and marked areas of color.
• Synthetic corundum will mostly have very pure, uniform colors. Fancy sapphires come in all colors, except red, which would be a ruby.
• Synthetic corundums, such as sapphire and ruby, are common on the market. They are often used as an alternative to mining practices and for purer, more colorful and affordable stones.

Recognizing Synthetic Quartz

• Natural quartz is generally pure. syntheses are difficult to identify without more extensive and costly scientific testing.
• The colors of synthetic quartz such as amethyst will be highly saturated and very clean.
• Synthetic quartz is common on the market, but its presence is difficult to quantify due to the difficulty of identifying it. They represent an alternative to mining practices and are generally affordable.

Frequently asked questions

1. What’s the difference between a synthetic and a natural gem? Synthetic gems are created in laboratories and have the same chemical, physical and optical properties as their natural counterparts, but are produced by controlling the growing conditions.

2. Are hydrothermal gems as precious as natural gems? The value of hydrothermal gems is generally lower than that of natural gems due to their artificial origin. However, their quality may be equivalent or superior in terms of purity and color.

3. Which gems can be synthesized by the hydrothermal method? Gems commonly synthesized by this method include quartz, emerald, amethyst, ruby and sapphire.

Sources

Handbook of Hydrothermal Technology: A Technology for Crystal Growth and Materials Processing
Materials science and process technology. Electronic materials and process technology. K. Byrappa, Masahiro Toshimura. 2001
Synthetic emeralds grown by richard nacken in the mid-1920s: properties, growth technique, and historical account. Karl Schmetzer, H. Albert Gilg, and Elisabeth Vaupel. Gems and Gemology 2016
Hydrothermal Synthesis of Sapphire R. A. Laudise and A. A. BallmanJournal of the American Chemical Society 1958
Hydrothermal Synthesis of Quartz Crystals A. C. Walker. Journal of the American Ceramic Society Volume 36 August 1953
Synthetic emerald: The confusing history and the current technologies K. Nassau. Journal of Crystal Growth Volume 35, Issue 2, 1976
Hydrothermal growth of gem stones and their characterization. M. Hosaka. Progress in Crystal Growth and Characterization of Materials Volume 21, Issues 1–4, 1991