I hope this helps. It sounds to me like she had made peace with her end and was preparing for the unknown. She placed it under her mattress to Keep away evil so she could cross over with ease. She was ready….
What you found is a protection stone called Black Tourmaline. Native American and African tribes, as well as some Australian aboriginal groups, used the stone to protect themselves against danger. The most protective of all crystals, Black Tourmaline supports the base chakra. It is said to encourage balance and neutrality, release stress, and discharge tension. It also known to dispel anxious feelings and negative emotions, creating a relaxed environment. This stone can apparently be used in homes as a protection grid.
I really hope this helps ease any worry. And helps your heart begin to heal. She is at peace. I recently lost my dad to pancreatic cancer. I know your pain and send my condolences and big hugs💕