The earth’s crust is made mostly from everyday rocks arranged in bumpy layers. We call them minerals and nobody knows how many different kinds there are. There are masses of granites and sandstones, which make useful building stones. There are whole hills of limestone, which we need to make iron and steel. And hidden here and there in the rocky layers, there are a few precious gem stones and sparkling crystals. The geode is one of these treasure troves and no diamond, ruby or emerald has a cleverer hiding place.

Chances are, you would pass it by without guessing its secret. You might mistake it for an ordinary pebble or for a lump of rock several feet wide. But if you picked it up, you would find that a geode is lighter than it should be   it weighs less than an ordinary stone. This is because it is merely a stony shell folded around a hollow inside. And its inside walls are lined with glassy crystals, semi precious stones or even with precious gems.

A geode may look like a pebble of milky white quartz, just like its neighbors lying on the beach' But it weighs less than it should, so you want to break it open. Careful now, for it contains a hidden treasure which you do not want to shatter. Its inside walls may be clustered with crystals of smoky grey quartz or glassy little bricks of rosy pink quartz, The hidden crystals may be violet colored amethyst, dark enough to be cut and used as precious gems.

We also find geodes hidden in softer calcite rocks, These stony jewel boxes too may be lined with pastel tinted crystals. Other geodes are lined with rainbow bands of gleaming agate. Others are lined with papery films of shiny copper or glimmering opal.

When you break open a geode, the treasure trove inside sees the light of day for the first time. For the pretty jewels were made from start to finish inside the rocky shell. The job was done by busy ground water trickling through the earth, drip dripping through cracks in the rocks and seeping through the tiny pockets in porous stones. This ground water dissolves all kinds of minerals as it licks and laps through the ground.

Here and there, a tumbling pebble leaves a hole, an empty pocket, in a slab of everyday rock. Ground water, rich with dissolved minerals, seeps into the hole and out again. Some of the chemicals may cling to the walls of the pocket. They have all the time in the world, so they arrange themselves in layers of glassy crystals. Much later, the jewel lined pocket is shaken loose from its slab of rock and left to lie on the ground as a geode.

The crystals in a geode are tinted with impurities from the ground water. Rose quartz is tinted with traces of titanium. The violet tones of amethyst are added by manganese. Though the jewels are well hidden, geodes tend to be neighborly stones. When you .find one, you are likely to find several more.