It is the icon that distinguishes espresso from the other methods of coffee preparation, and its thickness must be about 3 mm, equal to 10% of the total volume (25–30 ml).

When visually examined in the cup, the elements that highlight the quality of the mixture and the preparation are to be found in the colour of the crema (nuance and intensity), in its consistency (compactness and quality) and persistence (the amount of time it remains stable before disintegrating).

Its colour comes from the caramelization of sugars during the roasting process, while the consistency and persistence come from the emulsion of proteins during preparation, high molecular weight sugars, fats and other colloids.

Specifically, the crema is the result of the emulsion between gas (CO2) and oils that takes place inside the filter-holder cup during the extraction phase, by means of high pressure. During this phase the swirling movements of air and water are created at 88°–95°C (190°–203°F) and at a pressure of about 9 atmospheres which, through the previously pressed coffee panel, create the typical espresso crema in the cup.

A good espresso, prepared using a 100% Arabica blend, will be characterized by a compact and persistent crema, with an intense hazelnut colour and reddish and luminous reflections.

Mixtures also composed of percentages of Robusta quality, less rich in oils, will instead present a darker, more voluminous crema with less brilliant reflections. 

A crema of the beige hue just mentioned is an indication of coffee substations, while dark tones and reflections tending to mahogany are typical of an over-extracted espresso.