Adamantine 3/4-Plate

This suit of armor is reinforced with adamantine, one of the hardest substances in existence. While you're wearing it, any critical hit against you becomes a normal hit.

Three-Quarter Plate is a Medium Armor with a Base AC of 16, a Max DEX +2, and Min STR 13.  The weight is 45 lbs (assuming 6'0"/200 lb human).

In order to improve historical accuracy, a well as to allow for greater variety in attributes for design flavor, price points, etc., I have homebrewed new gradations of plate-based armor.  Primary sources confirm that historical plate armor varied greatly in the percentage of the body covered in plates versus chainmail worn underneath, as well as the thickness and quality of the alloys, the presence of reinforced layers over vital areas such as armpits and groin, and other aspects.  In order to reduce confusion, I preserved the 5e definitions, names, and attributes of "half plate" and "(full) plate," and simply inserted new forms around them.  They are as follows:

TYPE                                        CLASS                      MAX DEX         COST          MIN STR        AC       STEALTH                 WEIGHT
Half-Plate                                 Medium Armor             +2                    750               ---              15         Disadvantage              40
Three-Quarters Plate            Medium Armor             +2                 1,000              13             16         Disadvantage              45
Plate Mail                                 Heavy Armor                 +0                  1,250              14             17         Disadvantage              50
Full Plate                                  Heavy Armor                 +0                  1,500              15             18         Disadvantage              65
Heavy Plate                             Heavy Armor                 +0                  2,000              15             19         Disadvantage              55**

Each kind has an image as an example of the gradations here.  To summarize:

*  Half-Plate:  Consists of shaped metal plates that cover most of the wearer’s body. It does not include leg protection beyond simple greaves that are attached with leather straps.

*  Three-Quarters Plate:  Additional leg protection of a cuisse and poleyn, and perhaps some added gauntlets or helm or chainmail.  Often the plates only cover the front and part of the sides, leaving only the gambeson reinforced with chainmail for some unplated portions.     

*  Plate Mail:  The wearer has almost his entire body covered in chainmail but also has added thin plates, usually for the head, chest, hands, and thighs. The chainmail is superior too, with tiny ringlets that are riveted instead of welded.  While heavier than full and heavy/sealed plate, it is more affordable and easier to mass produce without custom measurements.

*  Full Plate:  This consists of shaped, interlocking metal plates to cover the entire body.  A suit of plate includes gauntlets, heavy leather boots, a visored helmet, and thick layers of padding underneath the armor.  Buckles and straps distribute the weight over the body.

*  Heavy Plate:  Represents the peak of armor technology (especially early 16th through 17th century) before firearms improvements forced rendered it impractical.  Because the plates fit so exactingly and even overlap at the joints, virtually no body part exposed.  Additional platelets typically reinforce the armpits, blade angles to the neck, and other vulnerable areas.  While quite hot under the armor, it is otherwise very comfortable because it fits so well and uses superior alloys that allow for thinner plates to be used without sacrificing protection; plus the heavy plate distributes the weight evenly across the body instead of upon the shoulders like chainmail that is used for other heavy armor. 

Half plate consists of shaped metal plates that cover most of the wearer's body. It does not include leg protection beyond simple greaves that are attached with leather straps.

Notes: Immunity: Critical Hits, Bonus: Armored Armor Class, Warding, Stealth Disadvantage