Quantifying and characterizing suspended sediment is essential to successful monitoring and management of estuaries and coastal environments. To quantify suspended sediment, optical and acoustic backscatter instruments are often used. Optical backscatter systems are more sensitive to fine particles ($<63 \mu m$) and flocs, whereas acoustic backscatter systems are more responsive to larger sand grains ($>63 \mu m$). It is thus challenging to estimate the relative proportion of sand or mud in environments where both types of sediment are present. The suspended sediment concentration measured by these devices depends on the composition of that sediment, so it is also difficult to measure concentration with a single instrument when the composition varies. The objective of this paper is to develop a methodology for characterizing the relative proportions of sand and mud in mixed sediment suspensions by comparing the response of simultaneous optical and acoustic measurements. We derive a sediment composition index (SCI) that can be used to directly predict the relative fraction of sand in suspension. Here we verify the theoretical response of these optical and acoustic instruments in laboratory experiments, and successfully apply this approach to field measurements on the ebb-tidal delta of Ameland Inlet in the Netherlands. Increasing sand content decreases SCI, which was verified in laboratory experiments. A reduction in SCI is seen under more energetic conditions when sand resuspension is expected. Conversely, the SCI increases in calmer conditions when sand settles out, leaving behind finer sediment. This approach provides crucial knowledge of suspended sediment composition in mixed sediment environments.