The gloss comes from an unusually smooth cuticle over the shell, and structural analysis has described it as among the most reflective in birds, closer to a glazed ceramic than to typical eggshell. Colour varies by species across turquoise, green, purple and near-black, but the shine is a family trait.
The puzzle is why. The eggs lie in a shallow scrape among leaf litter, not in a covered nest, and a bright glossy object is not obvious camouflage. Proposed explanations include a signal to attending males, a specular sheen that scatters light and confuses the outline, and defence against microbial invasion of the shell. None has clear support over the others.
What is settled is the parental system. Male great tinamous incubate clutches that several females contribute to, then lead the chicks alone, which makes the shining clutch on the forest floor his responsibility to hide.