B-But there’s no Kanoka…
Not everything green is a crystal, not every piece of metal with holes punched into it has a deeper spiritual metaphor behind it. The green here specifically does not reference chrysoberyls, tuurmalines, sphenes, or any other gemstones. It is an illustration of antidermis, the substance comprising the Makuta species, and has no other meaning beyond it.
Art (and product design) can exist absent from a metaphor in every element. We on this internet forum appreciate Bionicle for what it is, not what shapes we can contort it into. Sometimes there is no deeper meanings and significance beyond what the illustrators intended, in this case being a totally rad green disk embedded in metal with the island of Voya Nui inside to hype fans for the upcoming (as of 2005) line of Bionicle sets.
It doesn’t have to be anything more.