Agate:
Agate is a term applied not to a distinct mineral, but to an aggregate of various forms of silica,  mainly chalcedony.

No gemstone is more creatively striped by nature than agate, chalcedony quartz that forms in concentric layers in a wide variety of colors and textures. Each individual agate forms by filling a cavity in host rock. As a result, agate is often found as a round nodule, with concentric bands like the rings of a tree. The bands sometimes look like eyes, fanciful scallops, or even a landscape with trees.

Agate was highly valued as a talisman or amulet in ancient times. It was said to quench thirst and protect against fever. Persian magicians used agate to divert storms.

 

Many agates are hollow, and in these cases the last deposit commonly consists of quartz, often amethyst, which forms a crystal-lined cavity, or geode.

 

The agates are extremely resistant to weathering and remain as nodules in the soil or are deposited as gravel in streams and shorelines (Lake Superior agate).

 

Types of agate

A Mexican agate, showing only a single eye.  Included matter of a green, golden, red, black or other color or combinations embedded in the chalcedony and disposed in filaments and other forms suggestive of vegetable growth, gives rise to dendritic or moss agate (named varieties include Maury Mountain, Richardson Ranch, Sheep Creek and others). Dendritic agates have beautiful fern like patterns on them formed due to the presence of manganese and iron ions. Occasionally agate fills a void left by decomposed vegetative material such as a tree limb or root and is called limb cast agate due to its appearance.

 

 

Turritella Agate

 

Turritella agate is formed from fossilized Turritella shells silicified in a chalcedony . Turritella are spiral marine shells. Similarly, petrified wood, coral, and other remains as well as porous rocks can also become agatized. Agatized coral is often referred to as  Petoskey agate

Certain stones, when examined in thin sections by transmitted light, show a diffraction spectrum due to the extreme delicacy of the successive bands, and they are named rainbow agate.

 

Other forms of agate include carnelian agate , Botswana agate, Ellensburg blue agate, blue lace agate, plume agate (such as Carey, Graveyard Point, Sage, St. Johns, Teeter Ranch, ....), fire agate (which seems to glow internally like an opal) and Mexican crazy-lace agate .

 

 

Graveyard Agate