Mix baking soda and cornstarch in a bowl. Slowly add warm water, stirring until a clay-like dough forms. If sticky, add more cornstarch. Add coloring now if desired.

Press the mixture into silicone molds or roll out and cut with cookie cutters. Place shapes on parchment paper.

Let air-dry for 24–48 hours, flipping halfway for even drying. They should feel hard like plaster when ready.

Once fully dry, add 3–5 drops of essential oil to each stone. Let rest 1–2 hours to absorb fragrance.