Do diamonds have the effect of removing harmful sunlight, reflecting only the light that is good for the human body, thus making us healthy? No, that's not true: There is no scientific evidence that diamonds, both natural and synthetic, block harmful sunlight and "make the human body healthy."
The claim appeared in a TikTok video (archived here), published on November 30, 2023. The first half of the video talks about gold, while the second half focuses on diamonds. Translated from Japanese into English by Lead Stories staff, the video says:
Diamonds have the effect of removing harmful sunlight and reflecting only the light that is good for the human body.
(Source: TikTok screenshot taken on Fri Dec 27 11:13:07 2023 UTC)
There is no evidence indicating that diamonds block harmful sunlight. According to a newsletter (archived here) published by the Central Gem Laboratory, which is a company that grades diamonds, both natural diamonds and synthetic diamonds are made entirely of carbon, and have extremely high thermal conductivity, a refractive index of 2.417, and a dispersion rate of 0.044. According to an article by Mitsui Chemical Company (archived here), ultraviolet scattering agents, used mainly to disperse damaging UVA radiation and prevent it from reaching the skin, contain ingredients that reflect ultraviolet rays, such as titanium oxide and zinc oxide, but carbon is not one of them.
Some people believe that wearing certain types of jewelry can bring good luck as if they were amulets. This, though, is unsupported by scientific evidence. An article (archived here) published on January 29, 2021, on Yomiuri News online with the title "Don't be swayed by pseudoscience," describes this attitude as being related to spiritual beliefs, in contrast with science and rationality. This quote is translated from Japanese to English by Lead Stories staff:
There are many kinds of things in the human spiritual world, from fortune telling to amulets, astrology, blood type personality tests, and good luck items. Of course, if the person realizes that these things are a 'distraction' or 'relaxation' and gains peace of mind from them, then there is no point in avoiding them. Since the matter is related to a person's 'belief,' there will be people who find value in believing, in contrast to scientific rationality.
