There are basically two types of crystals Tag uses in its watches: sapphire and acrylic. Both types of crystals have benefits and drawbacks associated with their usage. Learning a little bit about both types of crystals and their functions is imperative to determining what kind of repairs to make and which type of crystal to replace the broken one with.
Decide which type of crystal your Tag watch has. Tag uses either sapphire crystals, which are expensive and the most scratch-resistant (they can only be scratched by diamonds and other surfaces that are equally as hard), or plastic/acrylic crystals.
Determine the type of damage your Tag watch has encountered. If your Tag has a sapphire crystal, it is very difficult to scratch it; however, it may be more inclined to shatter on impact than a plastic/acrylic crystal. If your sapphire crystal has shattered or if it has been scratched, you must have it replaced by a jeweler.
Repair the crystal yourself if feasible. If your Tag is equipped with a plastic/acrylic crystal, you may be able to buff out scratches yourself. Put a little toothpaste on the scratched area and buff lightly until the scratch is gone, or purchase a product made specifically for this purpose such as Crystal-Kleer. Acrylic/plastic crystals are resistant to shattering; however, if you do shatter your crystal you must take it to a repair shop to have it replaced.
Shop around for a certified Tag repair shop if necessary. Sapphire crystals usually cost over $100 to replace, and many places will not touch a Tag watch unless they are certified to repair it.