What I hope he'd think/say:
LT,
I had no idea that loaning you a random stone from my office could mean so much to you. But I'm glad it does. Seriously, keep it as long as you want. Yeah, it was a little weird to hear you talk about holding it until it got warm--seriously, you didn't realize that stones do that???--because that suggests you're holding it for a long time. But it's OK. I want you to feel safe and secure. It doesn't matter if I feel a bit puzzled about what makes you feel that way. If a rock does it, OK!

I hope you realize that I've come to really care about you. I really do want you to struggle less with all the thoughts in your head. I want you to find peace and happiness, whatever that looks like to you. I hope I can help you find that...even if I have to give you more stuff from my office!
Take care,
--Dr. T