This story is an idea that came to me in a dream - will be a multichapter :P
Hope you enjoy it!
The Doctor held a pickaxe in his hand and started abseiling down into the dark abyss.
"Oooh, Clara, you will love this. A rock that glows when you touch it. I know that you absolutely adore this kind of stuff." He softly murmured to himself as he gently tapped the surface of the translucent cliff with the pickaxe. He smiled as the brilliant light danced in front of the eyes when the pickaxe hit the cliff. Just as he acquired a rock of a satisfying size, the phone started ringing. He jumped at the sound, and with a sigh, he climbed back up to the TARDIS.
"Hello?" That sounded gruffer than he thought.
"… Hello? Is this the… Doctor?" A frail old lady's voice came through the line.
"Maybe. Who is this? How did you get my number?"
Through the line, he heard a sob.
"She said I needed to call you. She said that you were the best helpline in the universe."
"Who said? Who is she? Who are you?" The Doctor was slightly getting impatient.
There was another painful pause.
"She's very ill." The woman on the other line sniffled. "Doctor, please help my granddaughter. Save her. For me. Please."
"Who's your granddaughter? Listen, if you're going to be any vaguer, I'm going to hang up." He said, his patience wearing thin.
"It's Clara. She collapsed at work. She's dying and no one knows what's happening to her."
At the mention of her name, he dropped the phone. He sprinted around the console urgently, and landed immediately at the location of the call.
The Doctor opened the door to be confronted with an empty bed in a hospital room.
"You missed her." Dave replied, bereft. He was clutching onto Clara's green coat, with his eyes filled with tears. "She died five minutes ago."
The Doctor dropped the translucent rock, and it shattered into pieces on the cold, hard hospital floor.