  Fundamental research and technological applications of topological insulators
are hindered by the rarity of materials exhibiting a robust topologically
non-trivial phase, especially in two dimensions. Here, by means of extensive
first-principles calculations, we propose a novel quantum spin Hall insulator
with a sizeable band gap of $\sim$0.5 eV that is a monolayer of Jacutingaite, a
naturally occurring layered mineral first discovered in 2008 in Brazil and
recently synthesised. This system realises the paradigmatic Kane-Mele model for
quantum spin Hall insulators in a potentially exfoliable two-dimensional
monolayer, with helical edge states that are robust and that can be manipulated
exploiting a unique strong interplay between spin-orbit coupling,
crystal-symmetry breaking and dielectric response.
