Is the "gray" tincture intended? Gray is virtually only used by the US military heraldry people. It should be considered a colour rather than a metal, though I suspect it is considered to represent alumin(i)um.
Not wanting to hijack that thread, I thought it might be useful to clarify the use of gray in American military heraldry.
First, we need to distinguish between coats of arms and other devices. Regiments and battalions of the army, and units/formations of group size and above in the air force have coats of arms which are ordinarily supposed to conform to traditional heraldic principles. Patches, pins, etc. of divisions, brigades, commands, air force squadrons, and so on, are not bound by the same rules. The insignia of the 1st Infantry Division--a red Arabic numeral 1 on a olive drab background--is a case in point. No apologies for these--they're not arms and don't pretend to be.
The Navy is a law unto itself, even though it sometimes engages the Army Institute of Heraldry in designing ships' arms to a greater or lesser degree.
I. Silver Gray
The vast majority of appearances of "gray" in blazons of American military arms are in conjunction with the word "silver," i.e., "silver gray."
There are generally two reasons for saying "silver gray" instead of merely "argent." One is that blazons by the Institute of Heraldry (an Army organization, but one which is in charge of the production and procurement of heraldic items for all the services) are used to produce actual uniform items. Unlike heraldry in the wider world, this means that TIOH does not intend there to be any artistic license on how tinctures are represented; if they want the argent charge on the coat of arms to be represented as pale gray and not white or metallic silver, they often specify "silver gray," sometimes in conjunction with argent--"three mullets argent (silver-gray)."
The other reason for specifying "silver gray" in Army heraldry is that unit arms in the Army usually, although not always, make reference to the facing colors of the branch to which the unit belongs. Like facing and livery colors in Britain and elsewhere, these colors have never been constrained to the traditional heraldic palette. Just as British regiments have had facings in such colors as buff, orange, rifle green, bottle green, gosling green, pea green, and blue-green, U.S. Army branches have facing colors that include non-heraldic tinctures (such as buff) as well as specific shades of heraldic tinctures (dark blue, scarlet, jungle green, etc.). Silver gray comes into this as the main facing color of the Finance Corps (with golden yellow piping) and the piping color of military intelligence (with oriental blue facings) and psychological operations (with bottle green facings). In Finance Corps arms, silver gray is treated as a color, with Or as the metal; in MI and PSYOPS, silver gray is treated as a metal.
In the Air Force, "silver gray" is simply a specific shade of Argent and always used as a metal.
II. Gray as a more specific term for "proper"
Again, given that TIOH blazons are prepared for the purpose of producing standardized physical objects (uniform badges and patches, regimental and battalion flags, etc.), TIOH generally doesn't blazon things simply as "proper," since that leaves it up to the discretion of the manufacturer to decide what the natural color of something is. Accordingly, if a ship or a horse or a masonry wall is supposed to be gray, it is blazoned as such.
This is, as far as I can tell, much the same way that Eisengrau and Aschfarben are used in German heraldry and acier and cendreein French heraldry.
III. Gray as an additional tincture for fields and ordinaries.
This is rather rare in U.S. Army and Air Force heraldry, as far as I am aware. When it appears it is usually a reference to a National Guard regiment's lineage from a unit of the Confederate service. This is the case with about a dozen units or so. Other than that, the only cases I know of are the 258th Field Artillery (Gules two bars and in chief three mullets gray fimbriated Or) as an allusion the unit's historic designation, "Washington Grays," a couple of units that served with the 29th Infantry ("Blue and Gray") Division in WWII, and the 429th Support Battalion (Quarterly Azure and Gray a spear palewise issuant from base Argent surmounted by a fer de moline Or voided Gules), for which I see no particularly good excuse.
I hope that clarifies things.
