Here's an odd one that a friend and I couldn't agree on:
How does one make a masterwork artisan tool? Specifically, what skill is used? The same skill that the tool will be used for? What about the Alchemy lab for Craft (Alchemy)?
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Creating Masterwork Items
You can make a masterwork item—a weapon, suit of armor, shield, or tool that conveys a bonus on its use through its exceptional craftsmanship, not through being magical. To create a masterwork item, you create the masterwork component as if it were a separate item in addition to the standard item. The masterwork component has its own price (300 gp for a weapon or 150 gp for a suit of armor or a shield) and a Craft DC of 20. Once both the standard component and the masterwork component are completed, the masterwork item is finished. Note: The cost you pay for the masterwork component is one-third of the given amount, just as it is for the cost in raw materials.
So you have to craft both the item in question, and the masterwork component. This blurb leaves out the masterwork tool’s price, however. This is because it does vary somewhat, but for the generic tool, it’s 50 gp, and the base cost of a non-masterwork tool is negligible (unlisted, but it would presumably be well less than 1 gp in most cases; particularly expensive cases are called out separately, as with the Alchemy Lab).
Finally, there is a question of which Craft skill is involved. The answer to that is, whatever sort of thing you would need to make that sort of tool. This we determine mostly by comparison: when the masterwork item you’re making is a weapon, you use Craft (weaponsmithing) for the masterwork component; when it’s an armor, you use Craft (armorsmithing).
For an alchemy lab, which consists primarily of glass beakers, vials, and apparatuses, Craft (glassmaking) seems, at first glance, like the most appropriate choice, but you could easily make the argument that it’s actually Craft (alchemy) since the study of that craft may also cover the niche and specialized glassworking you need to make the apparatuses. I’d suggest that’s also better for the game; no need to make someone pay an unnecessary skill rank tax to be self-sufficient in their alchemy.
For many tools, Craft (metalworking) is probably the go-to, since many tools are made of metal. Some, however, could easily be Craft (stonecarving) or Craft (woodcarving), and Craft (weaving) or Craft (tailoring) might be just as big because some tools might be clothing you wear.
But the most important thing to remember is that masterwork tools are very much left undefined. Ultimately, the rules only specify that every masterwork tool gives a +2 bonus to some skill; it doesn’t even guarantee that every skill should have a corresponding tool.
Therefore, it’s up to the person who wants the tool to convince the DM that A. the tool would actually help in the desired skill, and B. could be made using the chosen Craft skill. In most cases, I’d recommend being pretty easy to convince on these; the bonus is minor and it’s often useful at low levels for characters to have a few things they can actually reliably use at basic levels (which the +2 does help a fair bit with, early on). But ultimately, it’s whatever the DM feels comfortable with. If every time the player uses the tool, you think to yourself “man, that thing is so dumb, how would it even help?” or “I can’t believe he made that with Craft (basketweaving), I mean, it’s not even a basket!” then you should probably bar it and make the player come up with something else.