Which substance is used to fill fractures in gemstones?

Resin is used to fill fractures in gemstones because of its ability to penetrate small cracks and voids effectively. It is a strong, clear substance that can be cured to form a solid, durable bond within the gemstone. This property not only helps to improve the appearance of the stone by making the fractures less visible but also can help stabilize the gemstone, preventing further deterioration.

In contrast, while wax may also be used in some instances to enhance the surface appearance of a gemstone, it does not provide the same level of durability or structural integrity as resin. Glue might not be appropriate for gemstone repair as it might not adhere well or could alter the clarity of the stone. Varnish, typically associated with wood finishes, is not suited for use with gemstones as it would not bond effectively or be clear enough to enhance the overall look.