It seems that hearts began to surface as a popular symbol in jewelry in the 15th Century in Europe, usually called a Witch’s Heart. This type of heart had more talismanic or protective properties and was not necessarily used as a love token, although could certainly have been.
In the 1600’s, we see the heart more frequently as the ever popular Stuart Crystal type of jewelry blended with the concept of the Witch’s Heart. It seems from examples of Stuart Crystal slides from the Mid-1600’s that the heart shape appeared more as we know it today. The only difference being a slight divet in the center, instead of the more pronounced cleft we see now.
Eventually, the heart shape went through it's many permutations in centuries past to became what we see today.
Now, Witches Hearts show up at auctions and at some higher end antique jewelry galleries, but once in a while, you can still see it turn up on someone’s finger. This is the ring that Princess Diana’s brother, Charles Spencer gave his first wife, Victoria Lockwood. Notice the pear shaped ruby and diamond instead of heart shape stones, harkening back to an earlier time in history. The crown of loyalty at the top still reigns supreme letting all of us "in the know" recognize this as a Witch's Heart.