A geode looks like an ordinary igneous or sedimentary rock, but the inside is often full of crystals! You can find geodes “in the wild” in some parts of the country, or at rock shops or science supply stores.

Watch our project video to see us crack open some geodes and find crystals inside:

What You Need:

What You Do:

  1. Place the geode inside the sock. This will help prevent rock fragments from flying and hurting you.
  2. Put the geode on a hard concrete surface, like a sidewalk, patio, or garage floor.
  3. Wearing safety goggles, hit the geode gently along the middle with the rock pick until it splits open. If you hit too hard, you may end up with lots of little pieces, so starting with gentle blows is best.
  4. Another method uses a chisel to split the geode, making it more likely to end up with two equal halves. Set the geode on the concrete, place the chisel in the middle, and tap it very gently a few times with the hammer. Turn the geode a quarter turn and do this again. Continue scoring along the circumference of the geode until you see a crack form all the way around, then pull the two halves apart. (This method works best with hollow geodes.)
  5. Examine the interior using a magnifier. Can you make out individual crystal shapes? How many different types of crystals do you notice?

What Happened:

A geode cracked open

Geodes are formed when there is a hollow cavity in solid rock. This cavity can form in several ways: by a gas bubble in a lava flow, by limestone being dissolved by an acidic substance, or by a shell not being filled completely with sediment when it is fossilized. When mineral-rich ground water gets into these cavities, the minerals can form crystals, depending on the temperature, pressure, and the amount of water. (Go here to learn more about crystals.)

Each geode is unique. Some will have large colorful crystal formations, others may have solid bands of quartz, and still others will have mineral deposits but no crystals formed yet. Sometimes a geode might be full of silt or sediment. This kind is called a “mud ball.” If you are selecting your own geodes, try to choose ones that seem light for their size; these are more likely to be hollow in the center and have crystal formations.

For a fun activity you can do with household materials, check out our Make Your Own Geode project!

How to Polish Rocks

polished rocks

If you’ve ever been to the beach, you’ve probably picked up rocks that are perfectly smooth and rounded. The ocean is a natural rock tumbler: as rocks are rolled by the waves and rubbed against sand and each other, their rough edges and corners are filed away. A rock tumbler can mimic this process and even take it a step further by adding polish to make the stones shine. Rock tumblers work much faster (4-6 weeks) than the ocean because their motion is constant and they use an abrasive substance called silicon carbide, which is much harder than sand.

Lapidary is the art of cutting and polishing gemstones. Rocks and minerals come in all different colors and patterns, and polishing them to a smooth shininess brings out their incredible beauty. Polished rocks are often used in jewelry.

What You Need:

Selection:

When selecting rocks to polish in a tumbler, there are several things to consider:

  • Size: You should include a mix of sizes in your tumbler, from 1/4″ to 1″. The smaller stones rub up against all the contours of the bigger stones, ensuring a uniform shaping.
  • Hardness: All the rocks in a batch should be around the same level on the Mohs hardness scale. The scale runs from 1 (talc) to 10 (diamond) and at each level a mineral can be scratched by something of the same or higher level, but nothing lower. If you tumble rocks of different hardness, the softer ones will be worn down too much and the harder ones not enough. If your rocks scratch each other equally, they are around the same hardness. The ideal hardness for tumbling is between 5-7. You can test the hardness level of your rocks using a set of Mohs minerals, or by trying to scratch them with a pocket knife (hardness 5.5) or a steel file (6.5). Harder rocks will take longer to tumble than softer ones.
  • Surface shape: Try to tumble rocks with similar surfaces. Rocks that already have rounded edges will need less time in the tumbler than rocks with sharp edges, so you should tumble them in different batches.
  • Beauty: Pick rocks that have colors and patterns that you like, especially when wet. (Polished rocks look similar to what unpolished rocks look like when wet.)

Tumbling:

Your rock tumbler will come with full instructions, but here are the basic steps to polishing a set of rocks:

  1. Fill the tumbler barrel 1/2-3/4 of the way full of rocks. The rocks lose a lot of their mass in the tumbling process, so if you start out with too few they won’t tumble properly later on. If you have the barrel too full, the rocks won’t be able to move enough to be shaped properly.
  2. Add the coarse grit and enough water to come up just to the bottom of your top layer of rocks. Together the grit, water, and rock particles that are worn away are called a “slurry.”
  3. Make sure the barrel opening and lid is clean and dry so it seals properly, then begin tumbling. Carefully open the barrel after about 24 hours to release any gas that has built up inside and to check that the slurry is forming. The slurry will look like a creamy dark gray mixture.
  4. Let the rocks tumble in the coarse grit for 6-7 days, checking periodically. After a week, remove the rocks and thoroughly clean them and the barrel of all grit. Warning: do not rinse the slurry down any household drain! It will harden like concrete in your pipes. Put it in a disposable container and throw it away.
  5. Sort through your batch of rocks and remove any that have cracks, chips, or pits. If they have any cavities where grit can get stuck, they will contaminate the future steps.
  6. Repeat these steps with the remaining stages of grit/polish, cleaning the rocks and barrel between each step. (For harder stones, you may need to repeat the coarse grit step if they aren’t shaped by the end of the first week. You need to repeat with new grit, because the grit loses its abrasiveness after a week.)
  7. By the end of 4-6 weeks you will have smooth, shiny, and beautiful stones!
  8. You can display them in a display case or vase, or use jewelry wire to make pendants and other jewelry.

Gemstones Science Lesson

A gem or a gemstone is any mineral that can be cut and polished for jewelry or other decoration. The most precious gems are chosen for their beauty, rarity, and durability. Semi-precious gems usually have one or two of these characteristics, but fall short in some area. Fluorite, for instance, is very beautiful but it is too soft and will scratch easily. Agate, quartz, and amethyst are other examples of semi-precious gems.

Diamonds are gemstones that are considered very precious, and for good reason. Though they are made of carbon, one of the most common elements, diamonds are generally regarded as some of the most beautiful gemstones. They are relatively rare, because much diamond is not of jewelry quality. Diamond mines usually have 1 part diamond to 40 million parts other rock, but a diamond high-quality enough to be in an engagement ring is the product of the removal and processing of 200 to 400 million times its volume of rock! The diamond’s strongest point, however, is its durability. It is the hardest substance found in nature, four times harder than the next hardest natural substance, corundum (sapphire and ruby). The grit in a regular rock tumbler wouldn’t have much effect on a diamond! It also has the highest melting point, and conducts heat five times better than the second best element, silver.