Geode Classifications

Quartz Geodes

• Geodes are generally classified according to their interior mineral crystal type. Quartz crystals are the most common type found inside geodes. The transparent to white rock crystals lining the geode's inner cavity are composed of silicon dioxide, the same mineral found in ordinary sand. A geode's internal quartz crystals terminate in hexagonal, pyramidal points that are larger as they approach the center of the geode. Quartz geodes may form in either igneous or sedimentary host rock.

Quartz Varieties

• Geodes may contain different colored varieties of quartz. The color of the quartz crystals is determined by the impurities of the surrounding substrate. Dissolved silicon dioxide combined with different compounds seep into the geode's hollow center to precipitate the growth of colored crystals. Iron oxide in growing crystals produces purple amethyst. Giant amethyst geodes associated with volcanic bubbles and tubes from Brazil are known as amethyst cathedrals. Other iron compounds cause yellow citrine geode crystals. Sometimes more than one color of quartz occurs in a geode.

Agate Geodes

• Agate geodes, featuring a wide range of bright colors, are popular with collectors. The stones are often sawed in half and polished to show off the bands and clouds of color. Agate is a type of cryptocrystalline quartz composed of tiny, tightly knit crystals. The different colored layers of fine-grained agate line the geode's interior in concentric circles. Sometimes the bands of agate give way to a crystalline center. Small agate geodes in the 1 to 3 inch range are called occo types.

Other Geodes

• Cracked-open geodes reveal a variety of mineral crystals, sometimes with a combination of several types. Calcite crystals of various colors are found as large cubic crystals or small pointed terminations known as dogtooth calcite. Celestine, or celestite geodes have transparent blue orthorhombic crystals, sometimes double-terminated, with points on both ends. Chalcedony often forms the hard outer rind of a geode. Inner botryoidal chalcedony has a smooth, rounded, bubbly shape. Geodes may contain crystals of dolomite, kaolinite, chert, malachite, iron pyrite, selenite and aragonite or other minerals.