Detailed Explanation
Freshwater Pearls implicates the following laws:
1. While a gratuitous bailee (unpaid trustee) or bailee for hire (paid trustee), assumes responsibility and liability by employing a legal act of acquisition [Choshen Mishpat 291: 2, 303: 1], a borrower assumes responsibility and liability simply by using the article [Nesivos 340: 1].
2. Grade of Liability
: 2.1. A gratuitous bailee (unpaid trustee) is liable to recompense for damages due to a)negligence [Choshen Mishpat 291: 1].
2.2 A bailee for hire (paid trustee) is liable to recompense for damages due to a)negligence, a
nd b)theft or loss outside of negligence [Choshen Mishpat 303:2].
2.3
A borrower is liable to recompense for damages due to a)negligence, b)theft or loss outside of negligence, or c)accidental causes [Choshen Mishpat 340:1].
Yet, a borrower is absolved from recompensing for damages incurred as a result of normal usage [ibid.].
3. Generally, a borrower (trustee) may not entrust a transportable deposit in the custody of a third party [Choshen Mishpat 342:1], even if motivated by kind intentions [Rambam, Kesef Mishna Sechirus 1:4].
Exceptions:
1) Adult Household Members. Barring obvious exclusions, it is generally assumed that a bailor (depositor) realizes that his/her deposit will be safeguarded or used by the trustee and/or his/her adult household members [Choshen Mishpat 291: 21].
2) A Frequent Trustee. Provided that the Grade of Liability is not reduced, (i.e. a bailee for hire may not entrust the deposit with a gratuitous bailee) a primary trustee may entrust the deposit within the hands of a secondary trustee whom the original bailor frequently entrusts articles of similar value and risk factors and whose ethical standard and damage-coverage ability (i.e. can still afford to cover his/her liabilities) has not since diminished [Rambam Sechirus 1:4].
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3) Implicit Circumstance: It is generally assumed that a bailor (depositor) realizes that circumstances arise when the primary trustee lacks the capacity to watch the trust and must appoint a deputy in his/her stead temporarily rather than leave the article unattended [Mordechai Bava Metzia].
4) In the Primary Trustee's Presence: Without increasing the risk factors, a primary borrower /trustee may allow a secondary trustee to use/watch the deposit if the article remains under the guard of the primary borrower [Kesef Mishna Sechirus 1:4].
Example: In her presence, a host may generally allow a responsible guest to read a book she borrowed from a neighbor.
5. A primary borrower who unlawfully appoints a secondary custodian is guilty of negligence, remains accountable to the bailor (depositor) and consequently assumes liability for ensuing damage or loss sustained in the third party's custody.
6. As the secondary borrower cognitively assumed responsibility for the article, he/she becomes accountable to the primary borrower.
7. The primary borrower must return the article or restore its value to the depositor unless he/she is able to bring valid testimony or swear that the loss resulted from normal usage and would have occurred regardless of whom was using the article[Choshen Mishpat 291: 26, 342: 1]. The depositor need not accept an oath from the secondary trustee. (Thus, it is conceivable for the secondary trustee to be absolved from paying the primary trustee; while the primary trustee must pay the depositor.)
Application
Cara borrowed freshwater pearls from Nina and may not indiscriminately loan them to the bride. Although, Cara was present during the wedding, the bride does not remain "under her watch" during the course of the ceremonies. In addition, as the bride is generally more active than the wedding guests, lending the pearls to the bride increases the risk factor. Thus, Cara was wrong and negligent for lending them to Cara and remains accountable to Nina. She must return the pearls, compensate their loss, or prove beyond reasonable doubt that the necklace was damaged as a result of its normal use.
Although, Leah did not request the necklace, she knowingly used it. Consequently, she assumes the liabilities and indemnities of a borrower and becomes accountable to Cara.
(As Leah had no reason to assume the necklace belonged to anyone other than Cara, she cannot even be held accountable for the damage she actively caused Nina [See Maseches Kesubos 34b]).
It is not uncommon for clasps to break during normal usage of a necklace. Yet, the burden of proof lies upon Cara due to her negligence. Thus, ieven if Leah claims that the clasp broke as a result of its normal usage, and as such will be absolved from paying Cara, Cara will remain liable to Nina as long as Cara cannot convince Nina of Leah's claim.
[Answered by The Fellow-Yesharim Research Center]
