N.Y. Dom. Rel. Law 236 (1980) (dual-classification)
Separate property is the narrow exception to the marital property rule, and is limited to the following categories: (3) gifts from third parties; Not all appreciation of separate property is marital. The courts have long distinguished between passive and active appreciation of separate property, and only active appreciation is considered marital property in which the non-titled spouse has an interest. Passive appreciation occurs when neither the titled spouse nor the non-titled spouse has made any active contribution that caused the asset to appreciate. Separate property co-mingled with marital property remains separate if it can be traced to its source
It should be noted that the spouse who contributed separate property may receive a credit for the amount of property contributed to the creation of the marital asset. Recent decisions have extended this concept to include the appreciation of the separate property as a credit to the spouse who contributed it. Once it can be shown that marital funds or labor have been contributed toward a nonmarital asset, the burden would then shift to the owner spouse to prove what nonmarital funds, if any, were contributed to the nonmarital asset. If the owner spouse is unable to meet the burden of proving what nonmarital funds were invested in the nonmarital property, then the entire appreciation would be subject to equitable distribution. Combining marital and separate funds in a single account generally makes the separate funds marital "only when it is impossible to trace the separate funds." The appreciation of separate property remains separate property (when it is pure and passive appreciation), Source-of-funds reimbursement is retroactive re-classification of marital assets into separate ones -- and these separate assets have been appreciating.