To be honest, with metal plates, a lot of the Technology unlocks use the same icon for that as the plate, so ideally, they should all be in the 128x128 format, not just 32x32.
I gave you a copy of one that looks like in game iron plate, along with 2 others, all from that final fantasy game.
So, A is iron plate, C is copper plate, and E is steel. The others all look like altered versions of these.
I still think you should arrange them by tiers, which would mean changing the base game graphics of Iron and Copper to match.
Alternatively, category.
For example... if you look at some of my chain, and how the metals change as the tier progresses...
Iron, Steel, Brass/Aluminium, Titanium, Nitinol is a good standard construction chain.
Iron, Steel, Invar, Tungsten, Copper Tungsten is a good "Heatproof" chain.
Copper, Bronze, Brass is a decent 3 tier copper based chain, which I've mostly just used for certain water processing equipment.
Iron, Steel, Cobalt steel, Titanium, Tungsten carbide is a good "Tool" chain. (I think)
Certain metals are rarely used for anything on their own, for example, Tin is mostly an alloying metal, but is also used in electronics. Zinc is also mostly an alloying metal, as is lead, Nickel, and Cobalt.
Cobalt and Lithium have a special purpose for batteries.
Gold and Silver (And Electrum) are "Precious metals", and are rarely used outside of electronic equipment, mostly due to being good electrical conductors, and resistant to weathering.
So as you can see, there's quite a few ways to group them there.
In my opinion, you should probably choose tier based icons for most of the construction materials, and don't forget that Silicon Nitride and Tungsten Carbide should be powders (Along with quite a few intermediate stages, or powdered versions of some metals).
Anyway, without being told what most of these plates in your example are, I couldn't guess. I'm guessing C goes Copper, Silver, Gold. E2 could be cobalt steel. B2 Is that distinct brass colour.
Though to be honest, I think that as a plate, these items should probably have a more uniform shape. Column B, and D look like they've either been formed, or not shaped yet, and those in F, G and H (Although I really like G), those shapes tend to be made to specifically be that shape as a strength reinforcement measure, so not ideal as a generic metal icon.
Which is one reason why I was looking back at those final fantasy icons I linked. Obviously the Iron, Copper and Steel plates (even though steel is a beam) do fit, and recoloured versions would work well for similar metals. we also have this one as a nice starting point for the tech icon
https://www.dropbox.com/s/tsjrmfp39psey ... n.png?dl=0
This one
https://www.dropbox.com/s/99qfguppf46jz ... n.png?dl=0 I think would work well as some sort of precious metal icon, like silver, gold etc, one good reason why I used it for cobalt. it could also be a good starting point for other non-construction metals like Solder, Cobalt, Zinc, Lithium and Silicon.
And this one
https://www.dropbox.com/s/mdvylcz3ok2wx ... n.png?dl=0 also a good one to use for non-construction metals.
I hope I don't come off too negative with these comments.