Kendall just posted this on Instagram:
Here is a close up, that she also posted on Instagram a while back:
The pendant reads: Blessed be the heart that bends, it can never be broken. You can buy it here. The black Swarovski pendant can be purchased here. You have to buy the pendant holder and chain separately, so it can get pricey. This also ships form overseas, so it can take a while to get to you, but it is SO cute, and SO worth it!