Transparent gem-quality varieties are known as peridots. Olivine alters readily to serpentine. It is found in ultramafic rocks and in the peridotite dikes in eastern and western Kentucky. Quartz is the hardest, most resistant mineral found in abundance in Kentucky. It is the main constituent in sandstones and geodes , and also occurs as vein quartz.
Crystals usually consist of six-sided hexagonal prisms capped by pyramids on one or both ends.
Quartz crystals are found in geodes that occur in several different rock types, particularly limestone. They are commonly recognized on the basis of their fibrous texture and granularity. The fibrous varieties include chalcedony, agate, onyx, and jasper, and granular varieties include chert and flint. Kentucky agate Agate has delicate and varying shades of color arranged in layers. In the typical occurrence the bands are irregular, curved, or in concentric patterns.