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Agates
Agate is a banded variety of Chalcedony, basically is a quartz material with a geode shape with concentrical bands inside in an irregular patterns due to a rapid sedimentation and cristalization of quartz
Physical Characteristics

 

Color: Any color
Luster: Waxy to dull
Transparency: Translucent to opaque
Crystal System: None visible (Trigonal)
Habits: Nodules in a wide variety of shapes, sizes and colors linning the interiors of geodes
Fracture: Conchoideal
Cleavage: Weak in three directions
Hardness: Less than 7
Specific Gravity: Less than 2.65
Streak: White
Other: Piezoelectricism
Occurrence: Mexico (Chihuahua, Sonora, Zacatecas), Brazil, Uruguay, Australia