Ground-state and finite-temperature behaviour of the mixed spin-1/2 and spin-1 Ising-Heisenberg model on decorated planar lattices consisting of inter-connected diamonds is investigated by means of the generalised decoration-iteration mapping transformation. The obtained exact results clearly point out that this model has a rather complex ground state composed of two unusual quantum phases, which is valid regardless of the lattice topology as well as the spatial dimensionality of the investigated system. It is shown that the diamond-like decorated planar lattices with a sufficiently high coordination number may exhibit a striking critical behaviour including reentrant phase transitions with two or three consecutive critical points.Copyright line will be provided by the publisher 1 Introduction Quantum Heisenberg models on geometrically frustrated planar lattices have enjoyed a great interest during the past decades especially due to their extraordinary diverse ground-state behaviour, which is often a result of mutual interplay between geometric frustration and quantum fluctuations [1,2,3]. From this perspective, geometrically frustrated quantum systems represent an excellent play ground for theoretical study of novel quantum many-body phenomena. Beside a rather complex ground state, which can be composed of several phases like the semi-classical Néel-like ordered phase, the quantum valence bond crystal phase or different disordered spin-liquid phases [1], quantum spin systems on two-dimensional geometrically frustrated lattices furnish a deeper insight into the quantum order-from-disorder effect [1,2,3], the scalar or vector chirality [4,5], as well as, the non-zero residual entropy that characterizes the macroscopic degeneracy of the ground state [1].