You don't have to be careless or clumsy to inflict scratches on a watch crystal. Most of the time you don't even know how it got there. Watch crystals made of synthetic sapphire, with a hardness of "9" on the Moh's hardness scale, are less common than in the past. Most crystals today are either hardened glass, known as mineral glass, or a form of acrylic plastic related to plexiglass. These materials have a lower hardness rating and scratch easier. The good news is, acrylic or glass watch crystals usually respond to simple home scratch-removal methods.
Common toothpaste contains mild abrasives such as fluorite and titanium. Registering "4" and "6" on the Moh's scale, respectively, these substances are of sufficient hardness to polish scratches from acrylic and standard glass watch crystals.
Choose a basic, no-frills white toothpaste. Gel toothpastes have a different formulation and do not offer the same abrasive properties. Specialty toothpastes for bleaching, tartar control or breath-freshening may have reduced abrasive content.
If the watch is not waterproof and can't be dipped in water after polishing, use masking tape to mask the metal parts of the case and bezel to prevent the toothpaste from getting into crevices.
Place a dab of white toothpaste on a clean rag. Wipe it onto the dry watch crystal. Rub in a circular motion, applying the most pressure to the scratched area. Continue to rub until the toothpaste is a dried haze.
Wipe the watch crystal with a clear rag and evaluate the results. Given the mild abrasive properties of toothpaste, it is likely additional applications will be necessary to achieve the best outcome. Approach it as an incremental process of removing the most superficial scratches first, then working down to deeper scratches.
Remove the masking tape, if applied. Use a soft toothbrush and water to remove toothpaste from crevices and metal parts of a waterproof watch.