Check the listing for details. Grape Agate Purple Chalcedony Tower- Specimen Quality- WYSIWYG. Condition: New. Listed at 33.00 USD. Grape agate, also known as botryoidal (purple) chalcedony or grape chalcedony, is a unique variety of chalcedony (variety of quartz) that forms in rounded, grape-like nodules. It is composed of silicon dioxide (SiO2) and typically exhibits a purple color, although it can also be found in shades of green, white, and blue.