“A” is for “Amygdaloidal” and “B” is for “Botryoidal Chalcedony”

Recently the Facebook Group I belong to, “New Zealand Lapidary, Rocks, Minerals, Fossils”, invited its members to submit photos each week of a stone, mineral or fossil in their collection that starts with a letter of the alphabet. It is currently the second week in the series.

For the first week, the following was my submission:

“A” is for “Amygdaloidal” – a stone from Gemstone Beach, Southland. An amygdale (amygdule) is an infilled vesicle [tiny hole originally formed by a gas bubble] on a volcanic stone, the term coming from Latin for almond reflecting the almond-shape of many such vesicles. Some of the ones in this stone are more circular than usually found. (So “amygdaloidal” is a term like “conglomerate” and “breccia”, denoting the way the stone has formed rather than the material it consists of.)

This week, I made this submission:

“B” is for “Botryoidal Chalcedony”. This stone was found in March this year on a small beach with sentinel seals at either end, south-east of Moeraki Village in North Otago. Thanks to Oliver Simpson for pointing me in that direction. In his 1954 book “The Pebbles on the Beach”, UK author Clarence Ellis writes of botryoidal chalcedony (p. 86): “Its surface is raised in rounded lumps suggestive of a bunch of grapes or a collection of soap bubbles.” (The Greek word for a bunch of grapes is “botrus”.) So “botryoidal” is a term relating to the surface shape of a stone, not what it is made of. Though there is a kind of relationship, to fibrous crystals. Clarence Ellis continues: “When broken, chalcedony is seen to be of a fibrous structure. Extremely thin fibres that look like hairs radiate out from the centre of each rounded lump to its surface.”

(For an account of the discovery of this stone, see this Post, Day 20. For a little more information on Ellis’s book, see this Post.)

The next Posts in this Series can be found here. An Index to the whole Series can be found here.