Your onyx jewelry needs special care most specially your black onyx rings because they are worn in one of the busiest part of your body, your hand, which is subjected to different kinds of substances like water, sweat, food, etc.
You should practice to keeping your black onyx jewelry clean. Take it off when you're going to handle any chemicals or any substances that will greatly affect the quality not only of the finish of your setting but also the stone itself. People have suggested the "put on last, take off first" method when putting on and removing your jewelry. Of course, you wouldn't want to lose them thus always make sure that they are in a secured place when you remove them. It can help if you would carry with you a small padded box where you can place your precious jewel just in case you are out and you really need to take it off.
Keep in mind that your jewelry is supposed to be worn not kept locked away somewhere. When you're wearing your jewelry though, take good care of it and not scratch or damage it. Onyx tips 6.5 to 7 in the Mohs (hardness) scale as compared to the diamond which is at 10 Mohs scale. This means that the onyx is not the hardest one in the bunch even if it looks tough.
If you need to store them, store them properly. Do not just pile your jewelry in your jewelry box. Some jewelry boxes have spaces with padding especially made to insert your rings. If yours doesn't have one, make sure that you keep each in velvety pouches which cases prevents the jewels from scratching and damaging each other.
Always be careful in using any type of jewel cleaner with your black onyx jewelry. It doesn't mean that if it is a jewel cleaner solution, you can already use it on your onyx. For example, submerging your onyx rings on ultrasonic cleaner might not be a good idea even if it is usually done to clean stones like diamonds, rubies and sapphires. Take note that each stone has a different composition. A cheap alternative is mixing a touch of ammonia on water. Unfortunately, this is also discouraged as ammonia is only recommended when cleaning diamonds, rubies and sapphires.
Have in mind that black onyx is a stone and is porous just like a rock. It absorbs the substance which it is submerged to and when this substance seeps into your gem, it will compromise the color of your stone as well as the purity of it.
For opaque gemstones just like your onyx jewelry, it is suggested that it should be wiped only with a moist cotton cloth. For black onyx rings (since it's has lots of grime), it should be soaked for one to two hours on warm water with mild soap and wiped clean with a damp cotton cloth first and then a clean dry cotton cloth afterwards. This is also suggested for necklaces are it is usually touches the skin too.
You should also avoid using polishing cloth. This is usually used to polish the metal of your jewelry. But do not use it on your onyx stone itself. This holds true the most when cleaning black onyx rings. You should be careful in cleaning that part which is in constantly touching the skin since it usually has engravings and dirt can pile up in its crevices. Keep it clean, you guys!