What is often used in the treatment of turquoise to enhance its durability?

In the treatment of turquoise to enhance its durability, the use of a polymer is a common practice. Turquoise is a relatively soft and porous gemstone, which makes it susceptible to damage and staining. By applying a polymer, a type of plastic resin, the stone's surface is sealed, reducing its porosity and improving its overall structural integrity. This treatment not only makes the turquoise more resistant to scratching and wear but also helps to minimize any discoloration that can occur from environmental exposure.

In contrast, heat treatment is not typically used for turquoise, as it can alter the stone's color and structure. Oil treatment is more commonly associated with stones like emeralds, where oil fills surface fractures to enhance clarity rather than durability. Water, while important for the care and maintenance of turquoise, does not provide the same protective benefits as a polymer treatment.

Therefore, the use of a polymer stands out as the most effective method for enhancing the durability of turquoise.