Leyline Compass

Do we have one, of course. Beautiful piece of work. Exquisite craftsmanship on the dragon about the border... No you cannot borrow it. You cannot use it, as a matter of fact based on this insistence you cannot even see it. Leylines are dangerous and best left alone.

— Jasper Cameron, Leader of The Bronze Ravens.

There is perhaps no greater addition to the advance of the study of magic than when the first leyline compass was discovered in the Hidden Castle in The Myrskyt Mountains. Being able to locate and map leylines has helped trace the history of magic across cultures and explain some of the more famous magical occurrences throughout the world, none of which we are here to discuss as the scholars at Silmere have several volumes on those subjects.
Instead we are here to discuss the compasses. To the best of our knowledge there are somewhere between 30 and 50 out in the world with varying degrees of functionality. Less than ten are able to track all of the 17 known leylines. Part of this is due to the impracticality of attuning the gemstones to each individual leyline before assembling the compass. A larger part is that for each additional leyline that is to be tracked a different type of gemstone must be used, or interference can occur. Destructive interference. Imagine a whine increasing in pitch as the gems vibrate in their socket until they start to explode.
Admittedly the same thing happens should you have even a single gem version when you reach the leyline. Again, and I CANNOT STRESS THIS ENOUGH, this is NOT a project for any of you to be attempting in your room or one of the warehouses you rent out for experiments. Yes we know about those, who do you think rents them out to you. No merchant would, you lot burned down 4 last year.
But back to the compass. Many consider the process of making one, if accomplished by a single person, qualification to be called an archmagus.
First, as already discussed one must acquire 17 gems of different types and sufficient quality to allow for magical attunement. As you learned in my class last semester, size and quality of the gem are inversely proportional to a point. so larger gems of a lesser quality can be substituted for a smaller gem. Keep in mind that for a compass to be useful you generally need to be able to travel with it.
After getting the gems, they will of course need to be cut and polished. For this particular application cut does matter, we will get to why later, but it is highly recommended you go with a cut that has a line of symmetry transitioning point-to-point. A marquise cut is perfectly suited to this, although there are others that work.

It is worth mentioning that while size is a substitute for quality, in an application such as the Leyline Compass you gemstones should be of a similar size to each other. The effects of aetheric capacitance on magical harmonics gets complicated. Plus there is the aesthetics of the things to consider.

Next you will need to attune each individual gem to its associated leyline. There are several ways that one can do this, the only general rule of thumb is the faster this is accomplished the more dangerous it is. The fastest method is to have someone with a working compass take you to the leyline in question and then channel a portion of the leylines magic through the gem until it is "magnetized." that is to say that in the leyline the gem will naturally orient to the flow of magic. This method, in all likelihood, will kill you and/or destroy the gem. DO NOT CHANNEL MAGIC FROM LEYLINES!
Far less lethal is to leave the gem in the leyline until it "magnetizes" naturally. Depending on quality, size, and cut this can take anywhere from 4-18 months. The signs of success are the same as those mentioned above. Should you find that your gemstone is not "magnetizing" it is likely that you are at a confluence or conjunction. That gem is largely useless for your purposes, but likely useful for other purposes. So.... sorry?
Once you have your gems attuned to the leyline all that is left is to set the gems, bifurcate them, and then assemble the compass. All that you need is some fey-silver, a clear slow viscosity suspension fluid, and a lensed case. Yes, bifurcate, as in cut-in-half. Point to point. perfectly, with magic. Hence making one of these being a qualification for archmageus. You cut the gem in half, rotate one side exactly 180 degrees, and mount it that way. That way when you ENTER the leyline it can split to align with the flow of magic instead of trying to torque itself out of its mounting and in all likelihood the case. Your other option to prevent this is to alloy fey-silver with mercury as your suspension and encase the entire thing in something like the lens of a krakens eye. That construction should allow the gem for a specific leyline to migrate to the center of the compass and align when one is on the leyline, but has the downside of needing an alloy only a single mage has ever produced and a kraken eye.
The most obvious use for the compass is the one that I have warned you about numerous times. DO NOT CHANNEL LEYLINE MAGIC! But there are several others. Properly marked, even as little as a 3 leyline compass can be used to determine your position anywhere on the planet. Or indeed your relative position. It takes the guesswork out of opening portals from other planes, although your best bet is still to just use an established crossing point. Leyline relations are not entirely consistent across planes and misreading coordinates could have you opening a portal to the bottom of the ocean, or into solid bedrock or thousands of feet in the air.