It is widely acknowledged that residual and sedimentary soils differ considerably because of how they formed. However, despite extensive advances regarding sedimentary soil, little is known about the small-strain stiffness of residual soil. This paper concerns granite residual soil from Xiamen in China, for which a typical profile is established by in situ investigations. How the stiffness varies with the soil current state, including effective mean stress, void ratio and strain, is established via systematic resonant-column tests on high-quality undisturbed and remolded specimens. Highlighted are the unique properties of the studied soil and how they affect the soil stiffness. It is found that cementation among soil particles plays a critical role in the soil stiffness. Although some existing methods can confirm the existence of cementation in the studied soil, they cannot quantify it. Therefore, a new parameter R_n is proposed to quantify the degree of cementation, with lower R_n corresponding to a more cemented structure. According to the proposed R_n, the studied residual soil in Xiamen has a different degree of cementation from that of some well-studied weathered materials, thereby necessitating separate characterization of this soil. This study improves the understanding of residual soil stiffness at small strain, especially the key effect of cementation in natural soil.