Rules: DMG 30.
Note the important message on DMG 30!: “Important: Players should never know the skill levels of jeweler-gemcutters!”
A hireling costs 100 gp/month to employ as a base cost.1) Jeweler-gemcutters who are not being employed on a monthly basis as a hireling but instead are being hired in a settlement in their own shop for example should simply have this base fee applied for the duration of the service. For example, if a month has 28 days, a hire of 5 days would cost 100 * (5/28), or about 16 gp.
In addition to the base cost, 10% of the value of the resultant gem or jewelry must be paid.2)
Some of the process is covered in the rules on DMG 30, but some things are left vague. To solidify:
The jeweler function cannot simply improve the value of an existing piece of jewelry. It is for crafting a new jewelry item out of materials, thereby increasing the value of the materials.
The time it takes to cut a gem depends on the target value of the gem (its base value if the cut succeeds):Ex
| Target Base Value | Time |
|---|---|
| 10 gp or less | 0.25 days |
| 50 gp | 0.5 days |
| 100 gp | 1 day |
| 500 gp | 2 days |
| 1000 gp | 4 days |
| 5000 gp | 7 days |