This article contains plot details from the latest book.
Ethertine is a rare substance that does not have many practical applications. It is mentioned to be rarer than Magsidian, but less useful and therefore much less valuable. It can be used on shades as a mind-control device, although it is unknown if the material has the same effect on other elves or any other species. Tam’s bonds, for example, were made from Ethertine, and therefore were able to control him. Ethertine casts a white, silvery glow, as seen when the dwarf king wears a crown made of the material.
Many assume that Ethertine is made of starlight and lightning, but as Nubiti points out that if it were true, "every thunderstorm with a break in the clouds would create more [Ethertine], and Ethertine is incredibly rare." (p. 713). Nubiti then alludes to her own thoughts that instead, Ethertine requires a specific kind of starlight: perhaps a type of quintessence.
When asked about the substance, Nubiti says that Ethertine has "been known to heighten emotion and dull inhibition for dwarves". Sophie then wonders if ethertine could be used to control elves' minds, and Nubiti agrees that it might because elves "are very sensitive to light—more sensitive than any other species."