Yes Max, I know that it must be quite impressive. I was just having a bit of a lark as usual 
Well i think to get any sort of inkling of what this could be, in paradigm at any rate, one would have to first determine what the title of custodian actually meant in medieval usage, since meanings do and have changed much in 700-800 years.
This site (useful for any Ars SG, no doubt) netserf.org/Glossary/f.cfm offers this meaning (found under Farm, Sheriff's): A custodian, on the other hand, directly accounted for all the revenues.
Then we have a further use of the term (not not necessarily mutually exclusive with the first) here castles-of-britain.com/castle38.htm to wit: Castellan: a person in charge of the castle, Custodian and again Custo: temporary custodian or governor of a castle or lordship.
It appears that unlike its modern usage (i.e. janitor or caretaker) it was much more a position of honour and responsibility entrusted often to a faithful slave or servant to guard AND manage a place or person(s) of great worth (castle, young nobles, treasury, etc.).
With this in mind I could posit that it might be a rather unique automata or just as easily an inanimate figure (more likely the former simply for the wow factor). As to its powers, as Cuchul suggests these could be myriad depending on what precisely is meant by "crystal" and what specific sort of "crystal" that might then be. Some possibilities construed from the matching of the item's title with likely chosen materials (also those presumably with the greatest wow factor for such a "master" work):
- Amethyst (wealth and mercantile): knowledge of the magus'/covenant's exact accounting to the least copper and charged with guarding the treasury. Might even BE the treasury itself through creative investiture of Muto and Rego Terram effects (serf's parma) to "absorb" and "dispense" coin as required?
alternatively (hearing): as a personal or covenant guardian against plots against the magus or covenant. Not likely to be aimed at other magi given the accolades bestowed upon Gudrun for its creation.
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Cinnabar (wealth, language, dragons?) A number of interesting possibilities here. In the first instance like unto the first example. The second bonus suggesting perhaps a capacity to interact with visitors/clients/etc. regardless of spoken tongue. The third suggests perhaps some powerful combat functions to protect a magus/covenant/castle against dragons (though not likely to be a much used option since ME isnt swarming with dragons).
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Clear Glass (Invisibility, Seeing through something). Useful options but not wholly satisfactory for the wow factor compared to more precious materials, not to mention again the unlikelyhood of widespread acclaim for a magical scrying device.
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Crystal (water-related effect): Certainly the most in keeping in with the device's title but in the Bavarian Forest not seemingly too useful unless it is perhaps a custodian of the forest imbued with weather enchantments.
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Diamond (versus demons): Now this strikes me as possibly the most likely both for the preciousness of the material (wow factor) as well as its usefulness with regard to demon protection/combat/slaying effects, especially in light of the targets which overly proud Verditii tend to be for demons.
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Emerald (calm, snakes and dragonkind) the first being again generally useful to keep covenfolk/castlefolk/villagers pacified and dutiful to the rightful masters. The second following the thinking mentioned above re "dragons". Not overly exciting perhaps even if the material is precious.
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Garnet (bonds of commitment, repel insects, strengthen body and mind, vigor): a pretty stone but again not much in the way of wow-factor compared to other likely possibilities. Generally useful first option for effects akin to that mentioned for "calm" just above. Second option is again useful to keep pesky insects (especially in the forest) at bay in an age long before window screens. Third and fourth options have much more general appeal especially for spellcasters who would enjoy a personal fatigue battery of sorts (provided perhaps a breakthrough beyond the canon fatigue exchange spell).
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Green Turquoise (necromancy): Coordinated colour choice for a covenant located in a desiduous forest but easy to lose the device amidst the background if it wanders, lol. That said, not (again) an effect category likely to gain widespread acclaim outside perhaps House Tremere (which I doubt the device made for, frankly).
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Rock Crystal (clarity): Another possibility in keeping with the device's title and perhaps quite useful for a number of Intellego effects (e.g. detect lies, sense extension, etc). Not overly precious material not overly exciting effects however.
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Ruby: (battle wounds, courage): Extremely useful to any covenant no doubt especially if the device is custodian for the covenant turb. A range of self evident effects here.
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Sapphire (Perdo Vim against spirits): Again useful for many covenants, most notably those containing any summoners. Likely to be sought after by the Donatores for quieting the restless dead. (Then again perhaps not)
Fairly singular in its focus though so again not overly likely.
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Turquoise (necromancy): as above.
All in all it would seem the most likely choices for material would (IMO) be
Diamond, Ruby, Amethyst, Garnet (the latter more for range of useful effects than material value) in that order. Speaking entirely hypothetically mind you, comments?
gives fingers a well-deserved rest