What does amygdaloid mean?
Amygdaloid is a term used above all in geology to describe volcanic rock — typically basalt or other lava — that is peppered with almond-shaped cavities known as amygdales. These cavities form when gas bubbles trapped in cooling lava are later filled by secondary minerals such as calcite, quartz, or zeolites, giving the rock a distinctive speckled appearance. The word derives from the Greek amygdalē, meaning almond, and can also be applied more generally to anything almond-shaped. In technical literature the adjective amygdaloidal is frequently used interchangeably, while amygdaloid itself may function as a noun denoting the rock. Though rarely encountered in everyday speech, the term is valuable for anyone reading about volcanic terrains, field geology, or the history of petrology, where amygdaloid lavas are a common and visually striking feature.
Of igneous rock: having an amygdaloidal texture, marked by almond-shaped gas cavities filled with secondary minerals; more generally, almond-shaped.
"An amygdaloid lava flow cropped out along the ridge, its cavities glittering with zeolite crystals."
In modern petrology the term 'amygdaloidal' is often preferred for the texture; 'amygdaloid' remains common in older literature and as a noun.
A variety of igneous rock, usually basaltic, characterized by an amygdaloidal texture of mineral-filled almond-shaped cavities.
"The prospectors traced the copper veins through bands of amygdaloid near the lake shore."
As a noun, 'amygdaloid' names the rock itself; the plural is 'amygdaloids'.
Used only when amygdaloid functions as a noun referring to specimens or varieties of the rock.
"Hand samples of several amygdaloids were laid out on the lab bench for comparison."
Geologists borrowed the Greek word for 'almond' to describe volcanic rock pocked with almond-shaped bubbles — so every amygdaloid basalt is, quite literally, stone full of stone almonds.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of amygdaloid
Amygdaloid comes from the Greek amygdalē, meaning 'almond', combined with the suffix -oeidēs ('-oid'), meaning 'having the form of'. The term entered scientific English in the late eighteenth and early nineteenth centuries as geologists systematized the description of volcanic rocks whose gas cavities resembled almonds. It shares this Greek root with amygdala, the name of both the almond and the brain structure, and with amygdalin, the compound found in bitter almonds.
Related word forms
How amygdaloid is actually used
Chiefly a technical term in geology and petrology; outside scientific writing it appears mainly in historical or descriptive accounts of rocks. The fuller form 'amygdaloidal' is often preferred for the rock texture, while 'amygdaloid' also serves as a noun naming the rock itself.
Easily confused with amygdaloid
The amygdala is the almond-shaped region of the brain involved in emotion, whereas amygdaloid describes anything almond-shaped — most commonly a type of vesicular volcanic rock.