Waco Gem and Mineral Club, Waco, Tx

10.5 Metamorphic Rocks


10.5 Metamorphic Rocks 

a) Making a metamorphic rock with clay

Metamorphic rocks are formed when pre-existing rocks (referred to as 'parent rocks') are altered by extreme heat and/or pressure. This often creates a whole new sort of rock with a new form and mineral structure.

To illustrate how pressure along with heat can change a rock into something new, you can do a demonstration with clay that Lowell Foster of the Ventura Gem and Mineral Society of California has shared:


Bars of clay of various colors: red, blue, yellow, white, etc. (Caution: Use clays that may be baked hard in an oven. Be careful in selecting your clay because not all clays are suitable for baking, and some synthetic varieties might actually catch on fire! That‘s because some synthetics are made from petroleum products. Most clays available in craft stores indicate on their labels whether or not they may be fire-hardened.)

Baking tray or pan

Toaster oven or your home oven


1. If kids twist and press together a bar of blue clay with a bar of red clay with a bar of white clay, the pressure and the twisting make a new clay with a swirl pattern. (Before they start twisting, have them break off and set aside small pieces of their original clay for comparison at the end of this activity.)

2. The more you continue twisting and mixing, the more the pattern and color may change, with blue and red combining to purple in places, or red and white turning pink.

3. If you now add heat to the equation by baking your new clay, you‘ll get a hard ceramic-like rock with a swirl pattern. You can bake specimens in your own home oven or in a small, portable toaster oven if it‘s capable of baking at 265° F for 30 minutes or so.

The tough new rock that comes out of the oven will be very different in color, pattern, and texture from the three individual soft pieces of clay your kids began with. In a similar manner, metamorphic rocks end up changed in color, pattern, and texture from their parent rocks by the combined effects of pressure and heat. Have kids compare pieces of their original red, blue, and white clay alongside the lump of twisted, mixed, and baked clay.

To conclude this activity, you can use thin strips of clay of many different colors stacked atop one another and apply pressure from the sides and/or twist and turn to make wavy patterns, or press holes into yellow clay and insert small balls or squares of red or blue clay to see what happens to their shapes when you then press down. Give clay to your kids, and let them get creative!

b) Collecting metamorphic rocks

Following are metamorphic rocks kids may be able to collect if they live in the right area of the country, or that they may be able to purchase from rock dealers or to trade through the mail via the AFMS Patricia Egolf Rock Pals program as a club project with kids in other AFMS/FRA clubs who live in areas where metamorphic rocks are common:

Gneiss (pronounced nice is a "high grade" metamorphic rock derived from various sources (e.g., granite, shale, conglomerate, etc.) that were subjected to intense heat and pressure, heat so high that the rock nearly melted to a magma, resulting in minerals that drew together in distinct banding patterns under the high pressure.

Greenstone is a fine-grained massive metamorphic rock with a dull luster that comes in varying shades of green; in California, it‘s associated with gold-bearing veins in the Mother Lode mining country.

Marble is limestone that has been altered through metamorphic action. Soft, easily carved, semi-translucent, and capable of taking a polish, it‘s often used by sculptors and builders. Marble comes in various forms, depending on the elements contained in its parent rock. For instance, limestone marble contains mostly calcium carbonate and may have interesting veining (or "marbling" with colors due to different mineral impurities. Dolomite marble had a parent rock of dolomite, which is similar to limestone, but with magnesium in addition to calcite as a constituent mineral. And mariposite (named after Mariposa, California, where it occurs in abundance) is a form of dolomite marble with a high green chromium muscovite mica content that gives it a distinctive green marbling.

Quartzite is a massive, medium-grained metamorphic rock with a sugary texture often derived from sedimentary sandstone.

Serpentine is a fairly soft metamorphic rock that may be waxy to the touch and has apple-green to black, mottled coloring that can look like serpent scales. It‘s the official California State Rock.

Slate is a "low grade" metamorphic rock (meaning it was subjected to only low heat and pressure) formed from sedimentary shale; it splits, or cleaves, in flat surfaces, and has been used as roofing shingles and blackboards.

Soapstone consists mostly of an impure, massive variety of talc. Soft, with a pearly sheen, it‘s a popular sculpting material, but has many other uses, such as in the manufacture of laboratory tabletops, firebricks, and electrical apparatus due to its resistance to heat, electricity, and acids.

Note: Kids can use this activity toward satisfying requirements for other badges, too: Rocks & Minerals (Activity 1.4) and/or Collecting (Activity 5.1).