First of all, how the heck could we possibly know that?

Secondly neither Iron nor Gold is a type of gemstone, so...

But diamond, lapis and pearl are not even gemstones 

Even if they aren't technically, they are close enough that most people would classify them as such.

Iron and Gold, not so much.

Diamonds are gemstones. Though made completely of carbon, they're only composed of a single element, has a uniform crystal structure, and found beneath the earth; it makes them minerals, it makes them gemstones. It is actually one of the four precious stones (the others being Ruby, Sapphire, and Emerald).

Lapis and Pearl aren't minerals, per se. Lapis is a rock composed of many minerals the most prominent one is lazurite, which gives it its blue coloration and pearls come from bivalves and some gastropods. But both are considered to be gemstones, amongst others.

But Bismuth seems to be an outlier of the lot. But since Bismuth is as a center piece in most jewelry rather than as the frame or holder (as Iron and Gold are used). It can be considered a gemstone, in an aesthetic sense.

Pearls are oyster poop.*

(which is probably why the Crewniverse made sure to tell us that SU Pearls are artifical creations rather than naturally-occuring ones)

not technically, but figuratively as it is a by-product of the way they eat*

Gold... maybe, iron...no. Bismuth isn't a type of gem too, but it counts as gem beacuse it's very similar to gems. Gold is jewerly metal and in various necklaces gems are putting in it, so maybe it will be something like diamonds' robot but smaller or a simbiote gem

Level [somethingsomething] canon, but at one point they did sketch out a character 'Concrete' (in one of their published artbooks) so I don't know how hard and fast the definition of 'Gem' actually is.

Especially considering Bismuth is an element mostly it's used in cosmetics and medicines.  It's not really a gem any more than gold or aluminium is ?‍♀️