This is an excellent African Green Moss Agate Cabochon, teardrop-shaped. Moss Agate has inclusions of manganese or iron that has grown into patterns similar to moss or lichen. It is translucent, with spots of dark green and sometime blue (in this case) or red.
Although this is a cab, you can also glue on "bails", or have the piece drilled to accept a pinch bail. Either approach will let you turn this sample into a fine pendant.