Indian jujube (Ziziphus mauritiana), also known as Ber, Indian plum, or Desert apple, has a unique taste depending on its ripeness.
When green and unripe, it’s crisp, mildly sweet, and a little tangy — kind of like a tart apple.
When fully ripe (yellowish-brown to reddish), the fruit becomes soft, sweet, and almost apple-like with a hint of date or caramel. 
It's an incredibly hardy fruit tree and thrives in hot, dry climates. Definitely a great addition to a rare fruit collection!