When English texts talk about the Three Sacred Treasures, they always talk about the Mirror, the Sword, and the Jewel. That is what I have always written.
It seems that this is wrong.
There was an article in Jinja Shinpō the other week about the accession ceremonies for a new Tennō, which referred to some ancient documents about the the jewel, in which there is a clear statement that there are eight jewels. The box containing the jewels has two boxes inside, one over the other, and each of the boxes contains four jewels.
How did this happen?
The fundamental thing is that Japanese does not normally distinguish singular and plural, so that almost all references to the jewels are ambiguous. Further, as they are never displayed individually, but only in the box, the number almost never comes up.
It is natural, in English, to think that there is only one, because there are three sacred treasures: one mirror, one sword, and one jewel. However, here again there is a translation problem. The standard Japanese is 三種の神器, which would be more literally translated as “The Three Types of Sacred Treasure”. That sounds odd in English, which accounts for the standard translation — even though it appears that it is actually wrong.
In my case, because I read the English before I read anything Japanese, I naturally interpreted the Japanese in light of the English, until I encountered something that actually contradicted the English. This took a long time; most Japanese do not know how many jewels there are either. I suspect that something similar happened to a lot of people, and thus the confusion persisted in English.
I had started to wonder whether the “jewel” was actually a string of beads, for three reasons. First, the small version that is hung on the masakaki in jinja is a string of beads with one or more curved jewels on. Second, the references in the myths to jewels are also to strings of jewels; the “jewel” is supposed to come from the legend of the stone cave, but the jewels that appear there are on strings. Finally, the name of the “jewel” is 八尺瓊勾玉, which means “Eight saka red jewels curved jewels”. “Saka” is a measurement of length, although the length it refers to is unclear (the same character is used for the ata, which appears to be about ten centimetres, and the shaku, which is about thirty centimetres). If this is literal, it sounds more like a string than a single jewel. This is indirect evidence, and you can stretch a point and refer to a single string of jewels as a jewel, so it hadn’t pushed me to change the way I wrote about it.
However, in light of the report from the ancient documents, the name might be literal, but mean “eight curved red jewels, each one saka long”. I somehow doubt that the Imperial Household Agency would let me look to confirm. In any case, it seems as though referring to “the jewel” is wrong; there are probably eight individual jewels, although there may be a string with eight jewels on, or maybe eight strings, each with a jewel on.
The obvious question to ask now is “how many swords and mirrors are there?”. In the case of the sword, the myths make it clear that there is one sword involved. It is used in myths, in contexts where multiple swords would make no sense at all. The mirror is not quite as clear, but Amaterasu Ōmikami does tell Ninigi no Mikoto to treat the mirror as her soul, which makes most sense if there is only one mirror, and the myths in which it was made also strongly suggest that there is a single mirror in the sacred treasures, although more than one may have been made initially, depending on the version of the legend you look at. Fortunately, I think I have not been wrong about that for years as well.
Taking everything into account, it seems that we should probably say that the sacred treasures of the Tennō are the Mirror, Sword, and eight Jewels.