Think twice before putting gems in an ultrasonic cleaner. Diamonds, rubies, and sapphires are fine, but many other gems may not be. When in doubt, leave it out. Diamonds, rubies, sapphires, and other single-crystal gems can be cleaned with jewelry cleaner to remove all films and add extra sparkle.

Opaque gemstones like lapis lazuli, turquoise, and malachite, require special care. Never use an ultrasonic cleaner and never use ammonia or any chemical solution. These gem materials should just be wiped clean gently with a moist cloth. These gemstones can be porous and may absorb chemicals, even soap, and they may build up inside the stone and discolor it.

The reason why these materials need more care than transparent gemstones is that these materials are essentially rocks, not crystals of a single mineral. Think about it: when you put a rock in water, it absorbs the water and is moist all the way through. A single crystal gem like sapphire will not absorb water: all the molecules are lined up so tightly in the crystal that there is no room for water to enter.

Diamonds are the hardest substance known but they grow and have a grain like all gemstones. If struck along a cleavage line a diamond can be split in two with a single, well-placed blow. Rubies and sapphires are the toughest gems but even they can chip, if hit sharply.

Opals also require special care. Never use an ultrasonic, never use ammonia, and avoid exposing opal to extreme temperature changes. Taking something out of the oven and then placing it directly in the freezer is an example of a temperature change that could affect the stability of an opal.