Alumina (Al2O3) thin films were prepared by RF magnetron sputtering technique using Al2O3 ceramic target. Effects of sputtering powers and oxygen gas mixtures were investigated and the optimized coating condition was applied on semi-precious gemstones. RF sputtering powers were varied to optimize the transparency of the films. Besides, the oxygen gas mixtures were also studied at the optimized sputtering power with a constant sputtering pressure. Optical and physical properties of the thin films were investigated using UV-Vis Spectrophotometer, FESEM, XRF, GIXRD, XRR including a microscratch tester. The Al2O3 films were highly transparent in the visible region in form of an amorphous phase with granular structure of the surface morphology. Thickness of the films decreased significantly with an introduction of the oxygen gas in the sputtering process but slowly decreased with further addition of the oxygen gas. Density of the film changed linearly with the variation of the oxygen gas mixtures. The semi-precious gemstones gained higher scratch resistance after the Al2O3 thin films coating. To enhance the scratch resistance and maintain the aesthetic appearance of the semi-precious gemstones, the most optimum deposition condition for the Al2O3 thin film coating was determined for the RF magnetron sputtering technique at room temperature.