Porous Stones: Such as pearls, opals, and emeralds, which can be damaged by ultrasonic vibrations. Certain Electronics: Items with loose or delicate components, such as hearing aids or non-waterproof watches. Delicate Materials: Items made of materials that can be easily scratched or damaged, such as certain plastics or wood. Items with Loose Inlays: Such as jewelry with glued-in gemstones, which can become dislodged. Vintage or Antique Items: Delicate items that may be weakened by age and susceptible to damage.