Use fake rocks in a garden for decoration or to conceal spare house or shed keys. Use fake rocks made from plastic latex in planters or indoor plant pots as well. You can make fake rocks easily from a clay mold and liquid latex casting compound. After you master the latex molding technique, you can make a variety of fake rocks and experiment with different sizes. Choose a palm-size rock with lots of definition to make a mold from which to make your first fake rock.
Measure the rock's height. Locate a box that is at least 1 inch wider, 1 inch longer and 1 inch deeper than the rock. Press oil-based, sulfur-free modeling clay into the box until the clay is 1 inch deeper than the rock. Sulfur reacts chemically with the latex molding plastic. Wet your fingers and evenly smooth the clay's surface.
Press the rock into the clay until just the very bottom of the rock protrudes from the clay. Because the rock will rest on the ground or in soil, you can make a one-part, press mold that is easy to make and use. Wiggle the rock slightly to lift the rock out of the clay.
Pour 1 cup of latex molding compound, also called liquid latex casting compound, mask latex or slush latex, into a container. Purchase latex at craft, hobby and mask supply stores.
Pour 1/4 cup of dark brown or dark gray latex-based paint into the latex. The Costumer's Manifesto website maintains that mixing the colorant, or latex-based paint, directly into the liquid latex is the only way to eliminate paint from eventually flaking off or drying and cracking. Mix the paint and latex well with a plastic spoon.
Add more paint, 1/4 cup at a time, until you reach the color desired for your fake rock. If you add more than 1/4 cup of paint, also add 1/8 cup latex compound per additional amounts of paint. Too much paint will compromise the latex.
Pour latex molding compound into the clay mold, filling the mold completely. Monster Makers website explains that latex casting compound is a room temperature setting latex that transforms into a solid product without the application of a heat source.
Allow the latex compound to congeal within the mold for about an hour. Turn the mold over and pour the excess latex into the latex container. An outer, hollow shell or skin remains within the mold. Allow this latex to dry for about 48 hours. You can make a solid fake rock by leaving the latex in the mold instead of pouring off the slush, but this can take a week to dry completely.
Remove the latex plastic rock from the clay mold by carefully peeling the clay away and lifting the fake rock out. Turn the fake rock over with the hollow core resting on a flat surface. Allow the fake rock to air dry until completely dry to the touch.
Dip a sea sponge into black latex-based paint. Dab the sponge onto the fake latex plastic rock to create textured surface color. Allow the paint to dry.