"A small fragment of a broken mirror."
According to the healer (or Dandelion, depending on Abigail's fate in Chapter I), Alina had a Mirror of Nehaleni before she died. It is not exactly clear if breaking the mirror brought on Alina's curse, or if the mirror broke as a result of the curse, but the mirror now lies in pieces strewn about the fields outside Murky Waters. In order to free Alina from her curse, Geralt must find the shards and reassemble the mirror as part of the Heat of the Day quest.
There are five shards to find in the fields. They are not difficult to find as each is guarded by a Midday bride and the glow can be seen from quite a distance. They can only be found during the day. The pieces are in the following locations: