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We report on the development of novel saturable absorbers for erbium lasers based on transparent glass-ceramics (GCs) containing a mixture of cobalt-doped β-willemite, Co²⁺:β-Zn₂SiO₄, and zinc oxide, Co²⁺:ZnO, nanosized (10-14 nm) crystals. The structure of the parent glass and GCs is studied by x-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and Raman spectroscopy. Variations of absorption spectra with heat-treatment reveal that Co²⁺ ions from the parent glass enter the crystals of ZnO and β-willemite. GCs are characterized by a broad absorption band due to the A₂4(F4)→T₁4(F4) transition of Co²⁺ ions in tetrahedral sites spanning up to ∼1.74  μm, relatively low saturation fluence, F_S=0.75  J/cm² at 1.54 μm, short recovery time, τ=830  ns, and high laser damage threshold, ∼14  J/cm². By using the developed GCs in a diode-side-pumped Er, Yb:glass laser, 0.77 mJ/45 ns pulses are generated.