The most appropriate Jesmonite product for a fireplace surround would be AC730.
Jesmonite AC730 is a water based micro-concrete composite material. AC730 is U.V stable and contains a blend of decorative aggregates and pigments to achieve a range of stone and concrete effect surface finishes.
We have a number of clients who create fireplace surrounds using Jesmonite AC730 in conjunction with x3 layers of Jesmonite Quadaxial Glass.
Majority of our clients would use a high grade silicone mould for ease of de-mould and fine detail replication. Once the AC730  laminate is cured (4 hour de-mould time) and de-moulded it would then need to be fixed or bonded onto a substructure or frame system.
Jesmonite AC730 isn’t a casting composite it is designed to create lightweight finished products which achieve a greater strength to weight ratio. I would recommend you laminate AC730 using x3 layers of Jesmonite Quadaxial Glass with recommended laminate thickness of 12mm.
However do not expose the material directly to fire/flames.
Please see the below formulae for working out AC730 laminate material coverage:
You will require 2kg of AC730 mixture per mm of material thickness into your mould.
Formulae: mould size X 2kg of mixture X mm thickness
Example formulae: 1m2 X 2kg AC730 mixture X 12mm = 24kg total AC730 mixture.