Despite the name, the gems.txt table holds gems and runes both — every socketable that grants a stat when slotted. 69 rows total: 35 gems (7 families × 5 quality tiers), 33 runes (El through Zod), and 1 Expansion engine marker row that divides Classic from LoD entries in the source file.

This is also the canonical source of the rune-name → rune-code mapping (Sol → r12, Jah → r31, Zod → r33) that runes and cubemain reference.

What's in a row

Column	Meaning
`name`	Reference field for the gem/rune name (matches the in-game display for gems and runes).
`letter`	String-table key for the letter shown in the item's tooltip when this is socketed. The engine resolves the key (e.g. `r12L`) to the display string ("Sol") and concatenates it into the item name. Blank for gems — only runes use this column.
`transform`	Color change applied to the item after this is socketed. References the `Index` value in `Colors.txt`.
`code`	The 3-char ID used to identify the gem/rune as an item. See Gem and rune codes below.
`weaponMod1Code` … `weaponMod3Max`	Up to 3 stats the socketable grants when slotted into an item with `gemapplytype = 0` (weapons).
`helmMod1Code` … `helmMod3Max`	Up to 3 stats when slotted into an item with `gemapplytype = 1` (helms or body armor).
`shieldMod1Code` … `shieldMod3Max`	Up to 3 stats when slotted into an item with `gemapplytype = 2` (shields).

Each mod group follows the same Code / Param / Min / Max pattern used everywhere else in D2R's affix tables. Code is a property name from Properties.txt (which in turn composes one or more itemstatcost stats with display logic). Param is a stat-specific tuning value (often 0 or unused). Min/Max are the value bounds — which are equal for socketables, since gems and runes don't roll.

Three mod groups, one row

The three column blocks (weaponMod*, helmMod*, shieldMod*) are why a Topaz behaves differently in a sword (lightning damage) than in a helm (MF) than in a shield (lightning resist). The selector is the gemapplytype column on the socketed item (defined in armor, weapons, and misc.txt):

`gemapplytype`	Mod block applied
`0`	`weaponMod*`
`1`	`helmMod*`
`2`	`shieldMod*`

Note the asymmetry: helmMod* covers both helms and body armor — there is no separate bodyMod* block. This is why a Sol rune in a helm grants the same 7 damage reduced as Sol in body armor.

Gem and rune codes

Gems

Codes follow a g<quality><family> pattern for the six colored / clear gem families, and a separate sk<quality> pattern for skulls:

Family	Letter	Chipped	Flawed	Normal	Flawless	Perfect
Amethyst	`v`	`gcv`	`gfv`	`gsv`	`gzv`	`gpv`
Topaz	`y`	`gcy`	`gfy`	`gsy`	`gly`	`gpy`
Sapphire	`b`	`gcb`	`gfb`	`gsb`	`glb`	`gpb`
Emerald	`g`	`gcg`	`gfg`	`gsg`	`glg`	`gpg`
Ruby	`r`	`gcr`	`gfr`	`gsr`	`glr`	`gpr`
Diamond	`w`	`gcw`	`gfw`	`gsw`	`glw`	`gpw`
Skull	(none)	`skc`	`skf`	`sku`	`skl`	`skz`

Two outliers in the quality letters: flawless Amethyst is gzv (not glv) and perfect Skull is skz (not skp). Engine quirks; ship as data.

Runes

Runes occupy 33 rows with codes r01 (El) through r33 (Zod). Their stats are a single mod per group — Sol gives +9 minimum damage in a weapon, +7 damage reduced in a helm or body, +7 damage reduced in a shield. No range, no roll.

Recipes and runeword definitions reference runes by their 3-char code, not by name. The full table:

#	Name	Code	#	Name	Code
1	El	`r01`	18	Ko	`r18`
2	Eld	`r02`	19	Fal	`r19`
3	Tir	`r03`	20	Lem	`r20`
4	Nef	`r04`	21	Pul	`r21`
5	Eth	`r05`	22	Um	`r22`
6	Ith	`r06`	23	Mal	`r23`
7	Tal	`r07`	24	Ist	`r24`
8	Ral	`r08`	25	Gul	`r25`
9	Ort	`r09`	26	Vex	`r26`
10	Thul	`r10`	27	Ohm	`r27`
11	Amn	`r11`	28	Lo	`r28`
12	Sol	`r12`	29	Sur	`r29`
13	Shael	`r13`	30	Ber	`r30`
14	Dol	`r14`	31	Jah	`r31`
15	Hel	`r15`	32	Cham	`r32`
16	Io	`r16`	33	Zod	`r33`
17	Lum	`r17`

Multi-mod gems

Unlike runes, gems can grant up to 3 stats per socket-slot type. Most gems use 1 of the 3 available mod slots — multi-mod gems are rare. A Perfect Topaz in a weapon grants +1-249 lightning damage (one mod, one slot used); in a helm it grants +24% MF (one mod); in a shield it grants +40 lightning resist (one mod).

Cross-references

runes — runeword definitions reference these rune codes in Rune1–Rune6.
cubemain — rune upgrade recipes (3 of rune X → 1 of rune X+1) reference these codes; gem-upgrade recipes (3 chipped → 1 flawed, etc.) similarly reference the gem codes here.
itemtypes — socket eligibility for gems/runes is gated by item-type flags here (MaxSockets1, gemapplytype defaults).
armor / weapons — gemapplytype and gemoffset on individual base-item rows determine which gem/rune mod block applies and which gem rows are eligible (filter by index).
Properties.txt — defines what each *Code resolves to (one or more itemstatcost stats with display logic).
Colors.txt — the transform column references the Index field here for the post-socket color change on items.