I like that this is a face swap and not a face copy. I feel like it needs a backstory about its use in a theater that eventually became infamous as the theater of cruelty. Perhaps this item has a twin mask, the tragedy one, which when it is combined has a sort of special effect. There could be a constant struggle in the seedy underground of the theater to try and hold both masks.
I fourth the desire to own one. It's simply so practical I don't feel it needs any real explanation. It's not a 5/5 post that makes you go "Woah... must... use..." but any DM or even a player can come up with a dozen clever uses for it. I especially like that it doesn't have that overwhelming aura of evil so many things have, it just has uses and 'misuses'.
I find this far superior to your Boo! article which seemed cheesy. I have to disagree with you on the point of not informing your players it is a horror game. Bait and switch is illegal for a reason. If the players did not want to play a horror game, then they have no choice but to leave or have you entirely change the game. Additionally, if they don't realize it is a horror game they will be far less cooperative in creating the mood because a fair portion of D&D is often in the out of game experience.
One idea shared with me was to actually give a picture of the gem to the player. If it's a semi-precious stone you most likely have one or can buy a small bag of them for a couple dollars. Gems are shiny. Really shiny. It's something you don't appreciate when the DM says "you have a 300 GP ruby" until you are looking at a $300 ruby. Beyond rubies, what does an adventurine look like? There are thousands of valuble stones you can use to add flavor.
I think the reason many GMs only go for gold and XP is the same reason you don't carry around a Rembrant three vassals and a cottage, it's inconvenient for the players and the GM. Most players just go and sell the art, at less of a value than they would get if it was just gold. Tracking NPCs is irritating for players (particularly those that don't have them) and in the adventuring lifestyle the die quickly. I've found that players who care about their characters will BUY clothes appropriate to their location to fit in with the locals. I do, however, like the idea of giving out potion recipes that have minor effects.
This is simply fiendish, a good puzzle. I always feel cheap when I have puzzles which revolve around the english language though, because in many RP settings there are dozens of languages. Who is to say Common is even something we can understand? Anyway, a minor quibble on a great work.