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Here are the current and older versions:
The 2.1 layout of materials (The 5 by 5):
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Grade 1 = C, Fe, Ni, P, S
Grade 2 = Cr, Ge, Mn, V, Zn
Grade 3 = As, Nb, Se, W, Zr
Grade 4 = Cd, Hg, Mo, Sn, Y
Grade 5 = Ru, Po, Sb, Te, Tc
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Previous (4-level)
Very Common - - Grade 1 = C, Fe, Ni, P, S
Common - -- - -- Grade 2 = Cr, Ge, Mn, V, Zn (+ As, Se, and Zr) - - - - (3)
Rare -- -- -- -- -- Grade 3 = Cd, Hg, Nb, Mo, Sn, W - - - - - - - - - - - - - (2)
Very Rare -- -- -- Grade 4 = Ru, Po, Sb, Te, Tc, Y - - - - - - - - - - - - - - (1)
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Using the old (4-level) version... every planet will still be 3-2-1... This has not changed... it just LOOKS different when applied to the 5-level mats new matrix.
The simple form is to just think of that LAST Line in the 4-level version... ONLY 1 OF THESE SHOULD EVER SHOW UP (*)
That leaves 5 other materials to find... and they have so far followed the old 4-level, 3-2-1 rule.
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Six (6) of the less common Mats to find in total.
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*(we have some verifications to do on the one or two planets where someone has said they've found more then one VR, but we are fairly confident and expect these to follow suit with the rest of the galaxy once we get out to them and give them a thorough going over

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MM's and OC2's (Metallic Outcrops) are indeed sometimes hard to find... but perhaps ... easier if you listen to the signals instead of looking for them, as the sounds they make start to appear well before you can resolve them on the scanner.