How is an icon different from an index in Charles Sanders Peirce’s theory of signs?

In the semiotics of Charles Sanders Peirce, signs are categorized based on their relationship to their referents. The distinction between an icon and an index lies in the nature of that connection:

• Icon: An icon functions through resemblance. It represents its referent by sharing physical or structural qualities with it, such as a road sign depicting falling rocks to warn of that specific hazard.1
• Index: An index functions through association or a direct connection to its referent. It does not necessarily resemble the object it signifies but is linked to it by cause or proximity, such as smoke serving as an indexical sign of fire.1

These categories are part of Peirce’s broader classification of signs, which also includes symbols—signs related to their referents purely by convention.1