Ultrasonic cleaners are safe for most gemstones, but not all. Some examples are: opal, tanzanite, emeralds, and a few more. These should be cleaned manually with a brand new, soft toothbrush. I say brand new because an old one will have toothpaste residue which is abrasive. You can set this toothbrush aside and use it exclusively for this purpose. I know there are a lot of people who will suggest using toothpaste to clean and polish jewelry, but over time, this will dull the finish, or luster on softer gemstones.
There are several smaller ultrasonic cleaners that are suitable for use at home ranging in price from around $40 to around $500. A cheaper unit is fine if you will be using it for a few minutes once in a while, as they are not designed for continuous use. They will over-heat and burn out. The more expensive models are really not necessary if you are using it for jewelry and other small items. Your best bet is in the $100 to $150 range. These machines are powerful enough to do the job, and will last you a long time. The tank capacity on these units are from around one pint to around two quarts, which should be big enough for any jewelry, and things as large as a pair of eyeglass.
An ultrasonic cleaner will work with plain water, but it will work much better if some kind of soap is used. A small amount of dish detergent like “Dawn” will work fine. “Simple Green is also a good choice, there are also several cleaners available that were designed for use in ultrasonic cleaners. Adding ammonia will also help, but it can discolor or damage certain gemstones. Ammonia will ruin the luster on pearls instantly. Also never use anything containing chlorine, like bleach, or certain laundry detergents and scouring powders. Chlorine can be harmful to some jewelry alloys, especially white gold.
Common white gold is an alloy of gold, copper, and nickel. Chlorine will react with the copper and nickel contained in the alloy, causing it to become brittle with repeated exposure. Avoid wearing your white gold jewelry in hot tubs and swimming pools. If you’ve done this a few times, there is no problem, but repeated exposure can cause prong tips to break off, resulting in lost stones, weakening neck chains, and clasps. Even yellow gold alloys are subject to this, but only slightly. White gold is the most vulnerable.
If you are spending a few thousand dollars on a diamond ring, spend a little extra and insist your stone be set in a platinum, or palladium white gold setting. Platinum neck chains are also available, though they will cost a lot more, but are completely impervious to chlorine.