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Affix Gems and Socket Rules
Four gem families, two tiers and typed sockets decide which affixes you can actually equip, including the one-way rule that trips up almost every new player.
Almost every affix you have came from a gem
Gear comes with a small number of built-in affixes, and past the early hours that stops being where your power comes from. The affixes that shape a build are carried by gems you socket yourself. Forty-four affix effects exist: defensive ones such as Aegis, Tenacious, Bulwark, Iron Helmet, Stoic and Spirit Shield; offensive ones such as Valor, Wrath, Sky Piercer, Fervid, Seeker, Smiting and Burst; utility ones such as Eloquence, Vitality, Elusive, Deft, Blessing, Curse and Wealth; plus a set of plainly named effects including Defence Penetration, Headshot Damage, Reduced Skill Stamina Cost and Increased Stamina Recovery Speed.
That last group is worth reading twice, because it contains the counterplay most players never equip. Affixes exist that cut the cost of dodging and that shorten how long you spend stunned, and they sit unused in warehouses while everyone argues about damage. If one specific thing keeps killing you, there is a fair chance an affix addresses it directly.
The reason this feels opaque is that the socket rules are never spelled out in game. They are not complicated, but they are strict, and a gem you cannot equip is worth nothing to you. Everything below is the complete rule set; the gem browser lets you filter the catalogue by family and affix once you know what you are hunting for.
Four families, two tiers, 312 gems
Gems come in four families: Agate, Amethyst, Moonstone and Peridot. Each family contains exactly 78 gems, for 312 in total, and draws from its own pool of twelve affixes. The tier decides how many of those affixes one gem carries. A Tier I gem carries a single affix. A Tier II gem carries two, and the Tier II sets are complete: every possible pair from a family's twelve-affix pool exists as a gem.
That completeness is why the numbers land where they do. Twelve single-affix gems plus every pair drawn from those twelve gives 78 per family, which is the entire catalogue in one sentence. It also means you never have to hope a particular combination was implemented. If both affixes live in the same family's pool, the Tier II gem carrying both of them exists somewhere in the game.
The constraint hiding inside that is the one that shapes real builds: a Tier II gem can only combine affixes from within a single family. No gem carries Valor and Blessing together, because Valor is an Agate and Amethyst affix while Blessing belongs to Peridot. When you plan a set of affixes, you are really deciding which families you will be shopping in.
What each family covers
Agate holds Aegis, Tenacious, Bulwark, Iron Helmet, Stoic, Brotherhood, Valor, Sky Piercer, Fervid, Focused, Distant Ward and Strife. It is the most defensive pool overall, with a few offensive options attached to the back of it. Amethyst holds Stoic, Valor, Wrath, Sky Piercer, Fervid, Seeker, Ranged, Fervor, Vitality, Curse, Unyielding and Resilience, which is where most straightforward offence lives.
Moonstone holds Aegis, Ethereal, Brotherhood, Spirit Shield, Wrath, Fervor, Seamless, Swift, Curse, Creation, Unyielding and Burst, mixing survivability with support effects, and it is the main family for a squad Seer. Peridot holds Tenacious, Ranged, Smiting, Eloquence, Seamless, Vitality, Elusive, Deft, Blessing, Wealth, Distant Ward and Strife; search it when you want utility, movement and loot value rather than raw output.
Several affixes overlap between pools. Stoic and Valor appear in both Agate and Amethyst, Aegis in both Agate and Moonstone, Tenacious in both Agate and Peridot, Distant Ward and Strife in both Agate and Peridot. When the gem you want is missing from the listings, check whether the affix also lives in a second family. Quite often the exact effect you need is available in a socket type you already have sitting empty.
Typed sockets and the one-way tier rule
Sockets are typed by family and by tier. Four typed Tier I slots exist, one per family, and four typed Tier II slots to match. A typed slot only accepts gems of its own family; an Agate socket will never take a Peridot gem, however much better that gem is.
Two universal sockets exist alongside them. One accepts any Tier I gem regardless of family; the other accepts any Tier I or Tier II gem. Those are the flexible slots, and a piece of gear that rolls one is worth more than its rarity suggests, because it is the only place you can put a gem you were not planning around.
Then comes the rule that catches everybody: a Tier I gem fits into a Tier II socket, but a Tier II gem never fits into a Tier I socket. Tier flows downward only. A Tier II socket is therefore strictly more useful than a Tier I socket of the same family, and a spare Tier I gem is never truly wasted, since at worst it fills a Tier II slot at half capacity until something better turns up. The flip side is that buying an expensive two-affix gem is pointless unless you already own a piece with the matching Tier II or universal socket. Otherwise it stays in your warehouse.
Why you never have the gem you need
Put the two constraints together and the frustration explains itself. A socket demands one specific family and one specific tier. A gem carries specific affixes and belongs to exactly one family. Matching a wanted affix to a free socket therefore needs the affix, the family and the tier to line up simultaneously, and with 312 gems in circulation the odds of the right one falling into your lap are not good.
There is also no rescue mechanic. Nothing currently fuses two gems into a better one, promotes a Tier I gem to Tier II, or breaks a gem down for parts, and no salvaging or disenchanting system exists or has been announced. A gem you cannot use stays a gem you cannot use until you sell it to someone who can.
A workable approach is to stop shopping for perfect gems and start shopping for the sockets you already own. Read your gear, list the free slots by family and tier, and buy only against that list. If a piece has an Amethyst Tier II socket, your search term is "two-affix Amethyst gem", not "best gem for my class". Every class also has an official shortlist of recommended affixes: Unyielding, Stoic, Bulwark and Tenacious for a Mercenary; Valor, Fervid, Ranged and Sky Piercer for a Blackarrow; Creation, Eloquence, Blessing and Brotherhood for a Seer. These lists quickly narrow a search that would otherwise be unbounded.
Making gems instead of waiting for them
Eight of the 165 crafting recipes produce gems rather than gear, and they are the reliable supply line. A Tier I Agate needs Agate, Resin and Seashell, and one craft returns three to five gems, which is why the station stays worth using even when auction listings look cheap. The Tier I Agate recipe shows its materials, its cost and its timing, and the other three families follow the same shape.
Tier II gems are a camp problem before they are a materials problem. They unlock at Workshop level 5, which itself needs character level 9 and Auction House level 4, so a player who has ignored their camp cannot make two-affix gems yet regardless of how many runestones they are sitting on. The recipes are heavier too: Tier II Amethyst wants Flawless, Fine and Improved Runestone, and the runestone chain is the first major hurdle.
If gems are your priority, plan the Workshop around that gate rather than pushing other buildings first, and check the full list on the crafting page before you sell any runestones. Crafts can roll a better rarity than ordered, but no reliable figure for how often has ever been published, so treat that as a pleasant surprise rather than part of the plan.
Living with gems on gear
Gems have no durability of their own, but their effects switch off when the item hosting them drops to zero durability. A broken weapon hits worse and stops supplying the affixes you paid for, which is a much larger loss and easy to miss in the middle of a raid. Repair before a serious run instead of after a bad one.
Socketing and unsocketing happen at the blacksmith and can also be done straight from your inventory or warehouse, so moving a gem between two pieces is not a trip you need to plan a session around. The place to be careful is selling: the auction house lists equipment and gems separately, and gear sold with gems still in it takes them along. Pull your gems out before anything goes up for sale, every time.
Think of a gem set as a semi-permanent asset that outlives individual pieces of gear. Armour is going to be lost in raids sooner or later. The gems that were in it, assuming you remembered to move them, are not.
Building a socket plan you can shop from
Work in this order. List every free socket you own by family and tier. Cross-reference that against your class's recommended affixes and mark the three or four effects you most want live. Then buy or craft only gems that satisfy both lists at once, and sell everything else to fund the next gem you can equip.
Check the plan against the rest of the build before you spend. An affix that improves blocking is dead weight without a shield in your hand, and one that improves ranged damage does nothing for a Shadowstrix. The equipment database marks which pieces come in socketed versions, so you can target the ones that carry the slots you are short of rather than the ones with the highest rarity colour.
Once the sockets are settled, the next question is which pieces deserve them, which is covered in gear rarity and what to upgrade. For how sockets sit inside the wider chain of class, weapon, skills and talents, start from how the build system fits together.