An embeddable, robust and cost-effective optical interferometric strain sensor with nano-scale strain resolution is reported in this paper. The principal structure of the sensor consists of an optical fiber, a quartz rod coated with a thin gold layer, and two metal shells employed to transfer the strain, orient and protect the optical fiber and quartz rod. The optical fiber endface, combining with the gold-coated surface forms an extrinsic Fabry-Perot interferometer. When the sensor is subjected to an external compressive/tensile stress, slide between the two metal shells will occur, resulting in a cavity length variation of the interferometer. A temperature compensation design is employed in the structure to minimize the temperature crosstalk of the sensor. The sensor was firstly calibrated, and the result showed that our prototype sensor can realize a measurement resolution of 30 nanostrain (n) and a sensitivity of 10.01 µ/µm over a range of 1000 µ. After calibration of the sensor, monitoring the shrinkage strain of a cubic brick of mortar in real time during the drying process was conducted. The strain sensor was compared with a commercial linear variable displacement transducer, and the comparison results in four weeks demonstrated that our sensor had much higher measurement resolution and gained more detailed and useful information. Due to the advantages of the extremely simple, robust and cost-effective configuration, it is believed that the sensor is significantly beneficial to practical applications, especially in structural health monitoring.