From my understanding, Root Chalice Dungeons are randomly generated - that is, no two maps are the same (generally speaking).
However, each dungeon generates a "key", so that other players can connect directly to it, for PvE or PvP.
Can I use this to re-create a dungeon I liked? E.g. if a Dungeon had a specific Gem or Item in it, can I use this key to re-create the dungeon?
Yes, you can use Chalice Glyph to get access to specific existing dungeon. That means you will see other players messengers and death spots if you play in online mode.
But your session (let's call it that way) will be unique to your character - all loot is in place, all bosses are alive, all lamps are undiscovered, etc. You can create more than 1 session at different tombstones in Hunter's Dream and progress will not be shared across them.
No resources are consumed if you use Glyph. However, you need to unlock similar dungeon first - acquire corresponding item (usually that means you need to finish non-Root version of dungeon) and generate dungeon with at least basic offerings.
If some dungeon is not active (no open sessions) for some time (1 week?) it will be removed: it will still be mapped to its Glyph, but on creation time one will get message that dungeon is inactive.
Generation and using Glyphs requires online mode, however you can switch to offline mode after session is already created.
If you are for runes and weapons, simply search for Glyphs where these items are confirmed to be - e.q. look at loot table in Isz Root Chalice and use corresponding Glyph.
If you are for good gems, it will require more work - gems has only some % drop rate from some bosses and regular enemies. Your goal is to find dungeon where desired gems could be dropped as early as possible. Then you recreate the dungeon till successful drop (pro tip- it's possible to copy-paste glyph using options button, you will get tired of retyping Glyphs very soon :) ). For example, Merciless Watchers boss on first layer of 5 depth level dungeon could be good place to farm 27.2% P. Attack gems. It's possible to use unfair (imho) technique to transfer saves on flash drive before boss fight, then restore saves if drop is poor.
There are tons of 'good' Glyphs available online, for example, some time ago I've discovered this spreadsheet.
