"It sounds like a lot of people would want it, so if it's missing it must be hard to find; how long do you think it'll take you?"
"It doesn't have to be hard to find, it just has to be out of reach of anyone who's actually tried. If it's in or near Sunnydale and there's a clue to be found, I'll have it by next week. If it's found its way to some other localized attractor of supernatural phenomena, it might take me a month. If someone actually managed to hide it in a hard-to-access location or erase all meaningful clues to its whereabouts, it could be longer. And if someone managed to destroy it I'll have to go with the fallback plan of figuring out how to reinvent it from scratch, which could really take a while."
"Things related to magic and demons and so forth that could happen near Sunnydale as opposed to in some other location usually do. There are more places like that, but the Sunnydale Hellmouth is the most obvious one on this continent."
"Oh, some of the crowd is interested in the Hellmouth or drawn by the size of the existing crowd. But plenty of demons and vampires and assorted magic users have no idea the Hellmouth is even here, and yet they still show up, in numbers vastly disproportionate to the size of the town. I'm not enough of a magical theorist to know if the amount of miscellaneous magic that clusters here without the obvious intervention of any intelligent beings is adequately explained by the presence of a great big gate to hell, but there's a lot of that too."
"Your tear is indeed an example of the kind of thing that shows up around the Hellmouth more often than can be reasonably explained."
"...I'm tempted to say 'why is magical theory such a fundamentally insane subject' but it's hard to explain what I mean when you have so little experience with local magic. You'll probably start to get the picture after you've been researching spells for a while."
Scrying techniques are many and varied and there is no clear reason why they should work the way the book claims they do.
"No good clues to the Gem's location in here," says Sherlock, closing the vampires book and wandering off presumably in search of another, better vampires book.
"How does anyone discover how to scry in a new way when it makes this little sense?"
"And you begin to understand what I mean about magical theory. The answer, as far as I can tell, is 'they guess wildly and are rewarded in no sensible pattern.'"