On page 150 of the D&D 5e Dungeon Master's Guide, there is an illustration of the Amulet of Proof against Detection and Location. The illustration depicts a closed eye, with a symbol above it, reproduced here to the best of my ability. I would like to know what this symbol is, and what its significance is, if any. It appears as though it might be an Alchemical symbol.
This is the symbol for the magic school of Abjuration. You can see it on a lot of different items in the 5E DMG, all of which function by protecting the user in some way: the Ring of Protection, the Scarab of Protection, and the Spellguard Shield, to name a few.
Since 2015, the symbols for magic schools have increased in visibility. They're used very explicitly in spell-related resources at D&D Beyond, with such prominence as to suggest the designers hope players will learn the symbols as handy visual aids, rather than semi-obscure easter eggs.
PLUS: Appendix B of Volo's Guide to Monsters includes illustrations of a transmuter's staff, an illusionist's staff, an evoker's staff, and a diviner's staff, each of which incorporates the symbol of its respective school in some way. If we add these to the Staff of Charming in the DMG, it starts to look like there's art for necromancy, conjuration, and abjuration staffs out there that we (or at least I) haven't seen yet.
You can see the symbols for all the schools in this image from the 3.5 PHB. In the image, the names of the schools are tiny scrawls, not very easy to read, but you can figure out which name applies to which symbol with a little bit of effort: Abjuration is the alchemical-looking symmetrical one at the top; Transmutation looks like an inverted Ц (Cyrillic 'ts'); Evocation is the crossed oval with a bunch of rays pointing out of it; Conjuration is the letter A with a ~ across the point; Illusion is the eye (simultaneously open and closed!); Divination is a hard-to-describe curly shape—I guess it looks a bit like a bifurcated stick you'd use to divine for water; Enchantment is basically the astrological symbol for Scorpio; and Necromancy is basically an Omega: Ω.
You can find some of the other schools' symbols on different items in the 5E DMG. The Staff of Charming is wrought in the shape of the Enchantment symbol, and the illustration of the Wand of Secrets has an alternate view just to point out that the symbol for Divination is etched on the tip. The Rod of Absorption displays four of them, and we can assume the other four are on the other side, since it can absorb any kind of spell.
This is one of the "portentous runes & glyphs" from the World of Greyhawk boxed set. Here's a pdf of the page on Greyhawk Online (go to the end). It means "uncertain, questionable." Also known as Gygax's rune system, they found their way into many AD&D adventures and such. Greyhawk wise, they appeared on p29-32 of the 1980 Folio, p17 of the Guide, p18 of the Atlas, and p17 of the '98 Player's Guide. They seem to be based on nordic/icelandic magical runes. One person claimed most of the runes can be found in Rudolph Koch's The Book of Signs but I cannot verify that.
There was a lot of this in early D&D. Larry DiTillio wrote an article entitled "The Glyphs of Cerilon" in Dragon 50. Dragon 69 had a series of articles and fiction about runes. "Pages from the Mages III" in Dragon 92 also introduces some new glyphs.