To summarize how the Chronicles describe the shape of Jinx, and its cause:

The shape of Jinx is unique; it was stretched into an egg-shape by Binary's tidal forces, and after its core cooled and became solid, it retained that shape as tidal forces pushed it away from Binary over billions of years. As a result it resembles an Easter egg, with colored bands of ocean and atmosphere. The Ends are 600 miles higher in altitude than the central sea level, which puts them well out of the atmosphere.

Detail of Jinx orbiting Binary, by John Schoenherr Copyright © 1974 by Condé Nast Publications

Science note: It is not possible for any planet as large as Jinx to be even remotely near 600 miles out of round. Earth's highest mountains are about 5 1/2 miles above sea level, and Jinx with its heavier gravity would have an even lower limit. The solid core is irrelevant; under enough pressure, rock or even solid iron will flow. Another problem with Jinx is that its sun, Sirius A, is a stellar type A, which burns through its stellar fuel in only a billion or so years. (Sirius B has already burnt out, having passed through the red giant phase to become a white dwarf.) This appears to allow insufficient time for Jinx to have a core which has completely cooled, and contradicts the implication that Bandersnatchi have been present on the planet for the 1.5 billion years since the end of the Slaver Empire.

Fanfix: We suggest Jinx has retained its egg shape because the molecular bonds have been artificially strengthened in its structure (or certain parts of its structure). The technique for producing this effect would be similar to that which produced scrith and General Products hulls, but since it was applied to natural materials, the product would be much weaker. Undoubtedly Jinx's solid iron core has been artificially strengthened in this manner.

Its mantle must also have been affected; perhaps the entire mantle, or perhaps the effect is restricted to certain elements, and only silicon and iron in the mantle are so affected. Repeated expansion and contraction due to natural surface heating and cooling degrades and ultimately destroys this artificial strengthening. Therefore, rock at or near the surface of Jinx lacks the abnormal strength and hardness of the interior, so it is no harder to dig a basement on Jinx than in normal bedrock.

A similar or identical technology may have been used to shape Mt. Lookitthat, although since the mountain is of recent geologic origin, it must have been shaped billions of years later than Jinx. It is possible Outsiders altered Jinx, but more likely another unknown, ancient species was responsible. Since Sirius is not old enough for Jinx to have formed and completely cooled during its lifespan, it seems apparent both Binary and Jinx were moved to their current location from another star. The Puppeteers are able to move their Fleet of Worlds while using artificial "sun" satellites to light and warm them, so clearly it is possible for aliens with powerful technology to have moved Binary and Jinx to their present location while keeping Jinx's biosphere from freezing or dying during the move.

It is tempting to suggest the motive for strengthening Jinx's structure was so it could more easily resist the tidal and inertial forces of acceleration during a move. That is, if uneven tidal forces during the acceleration would stress the planet, this strengthening would allow it to be accelerated up to speed more quickly without causing major earthquakes. However, Jinx's shape was clearly formed before the planet cooled— if it had been stretched into an egg-shape later, a series of enormous escarpments and fissures would ring the planet's surface. So the strengthening of Jinx's structure must have occurred billions of years ago, before it cooled and before tidal forces gradually moved it out away from the gas giant it orbits. (It is possible Jinx originally orbited a different Jovian world, but that is irrelevant to this hypothesis.) That an unknown species went to the undoubtedly enormous effort of strengthening Jinx's structure may suggest their method of moving the planet subjected it to some amount of stress, or it may suggest Jinx may have been repeatedly moved, for reasons unknown.