Steelix is a long, serpentine Pokémon with a silvery-gray body made up of rock-like sections, which can possess chips and scrapes from fights and tunneling. Three of those sections have long rock spikes coming out of them on either side. It has a massive head mainly made up of its large, wide jaw with sets of large square teeth inside. Along it bottom jaw are ridges: there are two pairs of ridges on a male Steelix, while there is only one pair on a female. Several small, square lumps cover the underside of its lower jaw. Its eye sockets are slightly deep set, and the eyes themselves are red. Its head has two long ridges going from the eyes up to the top of its head. The tip of its tail is conical with a blunt tip.
As Mega Steelix, its body becomes longer, almost doubles its mass, and gains more features. Mega Steelix's eyes turn blue and its lower jaw becomes more heavily armored with a round, curved shape. Additionally, a third pair of ridges appears on its bottom jaw, as well as two sharp, flat structures consisting of three conjoined projections on each side. The gray, rock-like spikes and its tail point have transformed into prism-like crystalline spikes; the segments from which the spikes protrude are much larger as well. In-between the head and individual spiked segments are smaller units, now adorned by a black spot with a blue hexagon on each side. Several metallic-looking scraps constantly rotate around its neck; these scraps react to Mega Steelix's movements and change position when attacking. The cells within its body, crystallized by the energy produced from Mega Evolution, are stronger than any mineral and able to withstand any temperature. It moves incredibly slowly, but its strengthened body is also impervious to damage, and it lashes out at foes without flinching. 
Steelix burrows up to six-tenths of a mile underground and can be found in mountains or caves. It easily chews through boulders with its powerful jaws, and its eyes are capable of seeing in the dark. Exposure to underground heat and pressures alongside its iron-rich soil diet have made its body harder than diamonds.
In Dealing With Defensive Types!, a Steelix belonging to Byron was used in the Gymbattle against Ash. It put up a tough fight, defeating Buizel, despite its type advantage and then fought against Chimchar. Despite its exhaustion from its earlier battle, Chimchar was able to defeat Steelix after a prolonged battle.
In Pokémon Adventures, Jasmine used a Steelix disguised as an Onix in the Gym Leader's Tournament of the opening ceremonies in the Pokémon League. Brock's Onix tightened its grip on the hidden steel Pokémon, but against the unusual hardiness of its evolution, it only hurt itself, and was eventually defeated after Steelix's stone "hide" scraped off and it was exposed. Jasmine explains that because it was a newly discovered species only so far uncovered by Crystal's capture, she tried to hide its identity in the hopes that it wouldn't be treated as simple eye candy for curious spectators.
A combination of Steel- and Ground-types, Steelix are the longest Pokémon yet to be discovered, reaching over 30 feet in length. Since they live so deep under the surface of the earth, the intense pressure has made their bodies stronger than diamonds. Evolved from Onix, Steelix is every bit as intimidating as its relative.
NA:With a body harder than diamond, this Pokemon is well suited to digging, using its powerful jaws to tunnel around. Steelix lives deep underground - even deeper than its pre-evolution, Onix. Was it pride that made Steelix prove itself able to go farther? It's hard to say. We tried to dig around for clues, but we got tired after only 10 feet...
PAL:Being harder than diamond, Steelix can crush any rock with its jaw. It can also see in pitch darkness. Its territory lies deeper than its pre-evolution, Onix, a full 1000m below the surface. Does it consider its Onix brethren to be "below" it, even though they're higher up? It's hard to say. We may have to dig a little deeper into the matter.
This means Mega Steelix has the highest Defense base stat and the highest single base stat of all Ground-type Pokémon, and is tied with Mega Aggron for highest Defense base stat and highest single base stat of all Steel-type Pokémon as well as the highest single base stat of all Mega Evolved Pokémon.
Steelix is shown floating above the ground in all console games, rather than, like its pre-evolved form, resting on the ground.
All Steelix in the anime have used the move Iron Tail (excluding cameos).