Determine whether the rock is igneous, sedimentary or metamorphic. Igneous rocks are extremely hard, sedimentary rocks are medium to soft with earth tones, and metamorphic rocks are medium to very soft with a variety of colors.
Feel the hardness of the rock. Granite, an igneous rock, is very hard, but still can be carved. Limestone and sandstone, both sedimentary rocks, are medium to soft. Metamorphic rocks like alabaster and soapstone are soft, making them easy to carve and add textured details. Marble is also metamorphic, but considered to be of medium hardness, and requires tools for carving.
Observe the rock's colors. Granite comes in grays, blacks, blues and browns. Limestone and sandstone each come in buff, grays, reds and browns. Metamorphic rocks like alabaster, marble and soapstone range from translucent white to dark black.