Agate

Agate
Moss agate pebble, 2.5 cm (1 inch) long
General
Category	Quartz variety
Formula (repeating unit)	Silica, SiO2
Identification
Color	White to grey, light blue, orange to red, black.
Crystal habit	Cryptocrystalline silica
Crystal system	Rhombohedral Microcrystalline
Cleavage	None
Fracture	Conchoidal with very sharp edges.
Mohs scale hardness	7
Luster	Waxy
Streak	White
Specific gravity	2.58-2.64
Refractive index	1.530-1.540
Birefringence	up to +0.004 (B-G)
Pleochroism	Absent

Agate is a kind of quartz and it is a mineral. Agate can come in many different kinds and colors. Agate deposits are primarily found in large rocks such as boulders. They take ~50 million years to form. It is characterized by colored band patterns inside various rocks. Often, agates are also accompanied by Amethyst deposits inside.^[1]

References

1. Streeter, Edwin W. (2014-07-07). Semi-Precious Stones - A Historical Article on Agate, Amber, Amethyst and Many Other Varieties of Gemstones. Read Books Ltd. ISBN 978-1-4733-9436-0.