From the d20 SRD:
Mithral
Mithral is a very rare silvery, glistening metal that is lighter than iron but just as hard. When worked like steel, it becomes a wonderful material from which to create armor and is occasionally used for other items as well. Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. Spell failure chances for armors and shields made from mithral are decreased by 10%, maximum Dexterity bonus is increased by 2, and armor check penalties are lessened by 3 (to a minimum of 0).
An item made from mithral weighs half as much as the same item made from other metals. In the case of weapons, this lighter weight does not change a weapon’s size category or the ease with which it can be wielded (whether it is light, one-handed, or two-handed).
How to approximate this, without item property fields for changing DEX bonus max, weight category, etc?
The easiest way I could see, would be for the crafting to actually spawn an armor that was in the next weight category, down, and make up the difference in armor modifier with additional enhancement bonus to AC. For example:
Current Mithral Breastplate: Breastplate with +2AC (total armor mod +7), 80% weight (not even close!), and 10% damage immunity (eh).
Reworked Mithral Breastplate: Scale mail with +3AC (total armor mod still +7), 70% weight (which would end up half of the breastplate).
You would gain, without other property mods, the 10% ASF reduction (because scale is 20% vs 30%), Max DEX +4 vs. +3, and -2 armor penalty vs. -5. This all pretty closely approximates all the benefits of mithral, except that it's still considered medium armor.
Current Mithral Scale: Scale with +2AC (total armor mod +6), 80% weight (not even close!), and 10% damage immunity (eh).
Reworked Mithral Scale: Leather armor with +4AC (total armor mod still +6), no weight reduction needed.
You would gain, without other property mods, the 10% ASF reduction (because leather is 10% vs 20%), Max DEX +6 vs. +4, and no armor penalty vs. -2. Weight category is light instead of medium. This approximates the benefits of mithral almost perfectly.
Heavier armors would base off of the breastplate, varying the AC enhancement bonus and weight reduction. +3/60% to equal mithril banded, +4/70% to equate mithril half plate, and (eek!) +5/80% to equate mithril full plate. All would be, as they should be, medium weight category instead of heavy.
Yes, this approach breaks the "Low/Mid-magic" barrier of present Regulith. Yes, it would make the mithril "heavy" armors hella expensive (and they should be). It would also affect some aesthetic in the fact that the torso models wouldn't match up for the type of armor it is.
If +5 for mithril "full plate" is too much armor bonus, then cap mithril at half plate, much like greensteel is capped at a breastplate.
Also, re: Adamantine armor, canonically:
Adamantine
....
Armor made from adamantine grants its wearer damage reduction of 1/- if it’s light armor, 2/- if it’s medium armor, and 3/- if it’s heavy armor.