The term 'Autumn Gemstone' can be solved by the four-letter word 'OPAL'. Opals are a type of gemstone that come in a variety of colors, including white, black, and various shades of pink, blue, and green. They are known for their unique play of color, which is caused by the diffraction of light through the stone's microstructure. Opals are often associated with autumn due to their warm, earthy tones.