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The green mineral dioptase Cu6Si6O18(H2O)6 has been known since centuries andplays an important role in esoteric doctrines. In particular, the greendioptase is supposed to grant the skill to speak with trees and to understandthe language of birds. Armed with natural samples of dioptase, we were able tounravel the magnetic nature of the mineral (presumably with hidden support frombirds and trees) and show that strong quantum fluctuations can be realized inan essentially framework-type spin lattice of coupled chains, thus neitherfrustration nor low-dimensionality are prerequisites. We present a microscopicmagnetic model for the green dioptase. Based on full-potential DFTcalculations, we find two relevant couplings in this system: anantiferromagnetic coupling J_c, forming spiral chains along the hexagonal caxis, and an inter-chain ferromagnetic coupling J_d within structural Cu2O6dimers. To refine the J_c and J_d values and to confirm the proposed spinmodel, we perform quantum Monte-Carlo simulations for the dioptase spinlattice. The derived magnetic susceptibility, the magnetic ground state, andthe sublattice magnetization are in remarkably good agreement with theexperimental data. The refined model parameters are J_c = 78 K and J_d = -37 Kwith J_d/J_c ~ -0.5. Despite the apparent three-dimensional features of thespin lattice and the lack of frustration, strong quantum fluctuations in thesystem are evidenced by a broad maximum in the magnetic susceptibility, areduced value of the Neel temperature T_N ~ 15 K >> J_c, and a low value of thesublattice magnetization m = 0.55 Bohr magneton. All these features should beascribed to the low coordination number of 3 that outbalances thethree-dimensional nature of the spin lattice.

Large quantum fluctuations in the strongly coupled spin-1/2 chains of green dioptase: a hidden message from birds and trees