Which plant has leaves that exactly look like stones or pebbles?

Short answer: One is the Pebble Plant (Lithops sp.) and another is the Lavender Pebble (Graptopetalum sp.) — but the likeness is not exact …

From Wikipedia article Lithops (Pebble plant (Living stone)):

Lithops is a genus of succulent plants in the ice plant family, Aizoaceae. Members of the genus are native to southern Africa. The name is derived from the Ancient Greek words λίθος (lithos), meaning stone, and ὄψ (ops), meaning face, referring to the stone-like appearance of the plants. They avoid being eaten by blending in with surrounding rocks and are often known as pebble plants or living stones.

I disagree with the Wikipedia botanist: The Lithops neither“avoid being eaten” nor “blend in” — the reason they used to thrive in the wild is that they tend not to be noticed by desert herbivores, as evolutionary selection pressure has eliminated all that did not happen to resemble hard-to-digest stones (“used to,” because thanks to plant collectors, it is nowadays rarely found in the wild — people are willing to pay top dollar for having these rock-like succulents in their gardens).

Pebble Plant image snagged from www.theclimbingfig.com.au article Lithops Marmorata ''Klip Pebble Plant'' seeds (Image copyright The Climbing Fig — Online Garden Store)

Lavender Pebbles image snagged from www.strangewonderfulthings.com article Graptopetalum amethystinum (Pachyphytum amethystinum). Photo Copyright 2004-2016 by Strange Wonderful Things. Original caption:

Lavender Pebbles is related to Jade Plant and Echeveria (Crassulaceae family). The chubby leaves are rounded at the tip, instead of pointy like many related succulents. The lavender or rose shades appear strongest in the newer leaves, while the older growth becomes more greenish.

See also

• Wikipedia — Graptopetalum (Leatherpetal)
• www.desert-tropicals.com — Lavender Pebbles