Squama vs. scaglia
The Italian words scaglia and squama both translate to "scale" when referring to fish and reptiles, but they are used in entirely different contexts. The word squama is primarily a biological and scientific term used for animal anatomy and dermatology. In contrast, scaglia is an everyday term used when preparing fish in the kitchen, describing the armor of fantasy creatures, or referring to a flake or shard of food and solid materials.
Squama
C2The word squama is the precise anatomical and scientific term for a scale of a fish, reptile, or butterfly wing. It is additionally used in medicine and dermatology to describe a biological scale or flake of dry, dead skin.
Il biologo ha esaminato la squama del rettile sotto il microscopio.
(The biologist examined the reptile scale under the microscope.)
Ogni squama sul corpo del serpente ha un motivo unico per mimetizzarsi nel bosco.
(Every scale on the snake's body has a unique pattern to camouflage in the woods.)
La malattia cutanea provoca la formazione di una fastidiosa squama sulla pelle.
(The skin disease causes the formation of an annoying scale on the skin.)
La squama di questa specie atlantica riflette la luce azzurra dell'oceano.
(The scale of this Atlantic species reflects the blue ocean light.)
Sulle ali della farfalla c'è una minuscola squama colorata che permette il volo.
(On the butterfly's wings there is a tiny colored scale that allows flight.)
Scaglia
B2The word scaglia is commonly used in everyday language to mean a fish scale, specifically when scaling a fish for cooking, or to describe a thick, armor-like scale of a fantasy creature such as a dragon. Furthermore, it broadly translates to a flake, chip, shaving, or splinter of solid materials like cheese, chocolate, stone, or wood.
Il pescivendolo usa un coltello apposito per togliere ogni singola scaglia dalla carpa.
(The fishmonger uses a special knife to remove every single scale from the carp.)
La spada del forte eroe non riuscì a scalfire minimamente la scaglia del drago.
(The strong hero's sword could not scratch the dragon scale in the slightest.)
Per finire il piatto serve una scaglia di formaggio stagionato o di tartufo.
(To finish the dish you need a flake of aged cheese or truffle.)
Mentre l'operaio tagliava la legna in giardino, una scaglia gli è volata vicino all'occhio.
(While the worker was chopping wood in the garden, a splinter flew near his eye.)
Il dolce della vetrina è decorato con una grossa scaglia di cioccolato bianco.
(The dessert in the display case is decorated with a large white chocolate shaving.)
Summary
To summarize, choose squama when talking strictly about biology, zoology, anatomy, or skin conditions. Opt for scaglia when talking about culinary fish preparation, the protective armor of fantasy reptiles, or when referring to a piece, flake, or chip broken off from hard elements like stone, wood, and food.