Why there are people cheating the Indian public by advertising that you should wear jewelry to counteract your karma is a small mystery – but the greater mystery is why the Indian public believes it. As Kaliyuga has advanced, there are practically no real vipra left even in India – therefore this sort of nonsense becomes so popular, and no one seems to have the knowledge or integrity to correct it.
Any first grade beginner dvija should know that the best way to improve your karma is to give money in charity and to sacrifice personal pleasures. Buying a diamond is nice, and no one minds looking pretty with a shiny jewel – but it is not going to modify your karma at all in any way – except to make your wallet skinnier. If you want what is best for you, take the money you would spend on a diamond ring and give it to a sincere vipra.
Then again the problem is that you will not easily find a sincere vipra these days. The public cannot even tell a real sadhu from a complete charlatan magician these days. So, best thing to do is find some humble soul who is always immersed in serving the Divine with mind, body, and words, especially by nama-kīrtana (the yuga-dharma), and respectfully give them the money you were going to spend on a diamond. THAT is “sukriti” (a deed that produces good karma), buying a jewel is shopping, not sukriti.
Please, don’t be foolish enough to ignore this advice.