Hello, Jim Buckley,
Stone used in this fireplace is Basalt. .basalt lava is a big part of Iceland. Now is ongoing eruption that produces basalt which melts at 1100 deg. centigrade, But now we are using the 1000 year old lava flow, melts at 1000 deg. The thickness of the slabs in the fireplace, 3 cm the hearth is 12 cm, and fireplace works well. As you notes there are three slabs in the walls. The opening is: 60 cm wide 80 cm high and 40 cm deep. No secrets, rock bursts rarely.
The slabs are gray from the nature, oil saturation makes stone black like send to you shortly photographs, as Count Rumford principle is involved.
All the best for you,
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