Speaking stone

A speaking stone, also called a sending stone, is a magical stone used to hold protected private conversations over distance. It is first seen in Changes.

Description
Harry uses a speaking stone to communicate with Ebenezar McCoy. The spell to activate it is fairly delicate magic.

Ebenezar McCoy crafted the speaking stones and has one identical to Harry's.

The magic used with the speaking stone is very much like the magic that Harry and Elaine Mallory used to talk to each other mind-to-mind. Harry uses that same magic again to talk to Susan in the Temple on the pyramid at Chichén Itzá.

a smooth piece of fire-rounded obsidian. Harry keeps it in a cigar box amid some other miscellaneous stones as camouflage.

Harry uses it with delicate spell-work inside his copper circle in his lab. When the spell activates, the lab vanished into inky darkness while the energy field of the circle he sat in became a visible cylinder shape of pale blue light from the floor to the infinite space. While talking, the voice echoes oddly. It's the same effect for the person on the other end. The two parties can see each other plainly.

Changes
In Changes, Harry uses it to communicate with Ebenezar McCoy. Harry asks Eben if the Grey Council can help get information for him on where the Reds may have taken the girl. Before Harry can tell him the truth that Maggie is his daughter, Ebenezar cuts him off by muting his voice and scolds him for only thinking of only one child, and tells him to meet at Toronto safe house in twelve hours, then hangs up.

Later, McCoy calls him on the Stone while Harry Dresden is in the Leanansidhe's limo with the rest of his team on their way to the Ways to go rescue Maggie from Arianna Ortega and the Reds at Chichén Itzá. The team thinks Harry's acting odd when the call comes in. Eben starts scolding Harry for missing the meeting and for what he percieves as foolishness. Harry fimally manages to tell him about Maggie being his daughter and that Arianna Ortega plans a Bloodline Curse to kill her, Susan and himself. Eben softens to Harry's pain and sees the gravity of the situation and that it's far bigger than being just about Harry and Maggie. He tells Harry to go get his girl.

External references

 * Obsidian Healing and Magical properties, Talismans and Amulets
 * Black Obsidian Stone Cleanses Negative Psychic Smog From Aura
 * Black Obsidian
 * Metaphysical Properties of Obsidian ~ LightandLoveReiki.com
 * Obsidian - Wikipedia