Porous rocks have the ability to soak up fabric dyes or food colouring when heated. The heat will open the pores and allow the colour to penetrate the rock's surface --- afterward, you can clean and polish them too. You may want to research or experiment with various rocks to determine whether or not they can be dyed --- igneous rocks, such as basalt, pumice and scoria, work well.
Clean your rocks thoroughly under running water. Dry with a napkin or cloth.
Place rocks in the oven. Heat to approximately 93.3 degrees Celsius.
Prepare your dye solution according to the package's instructions. Use heated water so that the hot rocks do not crack or split when placed in the dye.
Remove rocks carefully from the oven with tongs. Place them in your heated dye solution and cover.
Leave rocks submerged in the dye overnight. Remove and rinse.