Guide
Socketing gear

Sockets hold gems. A socket is added by an NPC jeweler: dropped gear uses one chance ladder, player-crafted gear another. Each item's own ladder lives on its item page — the arrays below are read straight from those rows.
Dropped gear
Chance for the 1st, 2nd, 3rd… socket on a dropped piece (drop_socket_prob):
Guardian Corselet
1: 95% · 2: 3.5% · 3: 1% · 4: 0.50%
Blessed Chain Hauberk
1: 90% · 2: 7% · 3: 2% · 4: 1%
Crafted gear
Crafted pieces roll a longer ladder (make_socket_prob):
Guardian Corselet
1: 93.6% · 2: 3.9% · 3: 1.5% · 4: 0.89% · 5: 0.10%
Blessed Chain Hauberk
1: 87.4% · 2: 7.8% · 3: 2.9% · 4: 1.7% · 5: 0.19%
What a socket rolls
A new socket rolls a stat bonus from the equipment-addon pool; examples from that table:
addon 221 — Phys. Def. +15~25% (15%~25%)
addon 222 — Phys. Def. +25~35% (25%~35%)
addon 223 — Phys. Def. +35~45% (35%~45%)