Which synthetic gem production method is characterized by low cost and high volume?

The method characterized by low cost and high volume in synthetic gem production is flame fusion. This technique, also known as the Verneuil process, involves melting a powdered form of the desired material and allowing it to crystallize as it cools. Because of its straightforward setup and the speed at which it produces gems, flame fusion is an efficient method that leads to a high output of synthetic gems at a relatively low cost.

The simplicity of the flame fusion process means that it requires less elaborate equipment and less time compared to other methods, which often involve more complex conditions and longer growth times. This makes it particularly appealing for mass production of gemstones, such as synthetic rubies and sapphires.

In contrast, the other methods listed typically involve more intricate processes that can be more expensive and time-consuming. Flux growth and hydrothermal growth, for instance, require carefully controlled environments and extended growth periods, making them less suitable for high-volume production. Pulled crystals, while useful for creating certain types of gems, also involve a level of complexity and cost that does not lend itself to high-volume output like flame fusion does.