Emerald is 7.5 to 8 on the Mohs scale and has fair to good toughness, making it a stone that requires more care in wearing than ruby or sapphire. Even so, emeralds are beautiful stones for all types of jewelry and with proper care will last for generations.
About 90 percent or more of emeralds are fracture-filled. Since the great majority of fashioned natural emeralds contain filled fractures, it’s risky to clean them ultrasonically or with steam.
Ultrasonic vibrations can weaken already-fractured stones, and hot steam can cause oil or unhardened resin to sweat out of fractures. Using warm, soapy water with gentle scrubbing is the safest way to clean emeralds.