The associates (unit multiples) and the complex conjugate of any Eisenstein prime are also prime.
An Eisenstein integer z = a + bω is an Eisenstein prime if and only if either of the following (mutually exclusive) conditions hold:
- z is equal to the product of a unit and a natural prime of the form 3n − 1,
- |z|2 = a2 − ab + b2 is a natural prime (necessarily congruent to 0 or 1 mod 3).
It follows that the square of the absolute value of every Eisenstein prime is a natural prime or the square of a natural prime.
In base 12, the natural Eisenstein primes are exactly the natural primes ending with 5 or 3 (i.e. the natural primes congruent to 2 mod 3). The natural Gaussian primes are exactly the natural primes ending with 7 or 3 (i.e. the natural primes congruent to 3 mod 4).
The first few Eisenstein primes that equal a natural prime 3n − 1 are:
Natural primes that are congruent to 0 or 1 modulo 3 are not Eisenstein primes: they admit nontrivial factorizations in Z[ω]. For example:
- 3 = −(1 + 2ω)2
- 7 = (3 + ω)(2 − ω).
Some non-real Eisenstein primes are
- 2 + ω, 3 + ω, 4 + ω, 5 + 2ω, 6 + ω, 7 + ω, 7 + 3ω.
Up to conjugacy and unit multiples, the primes listed above, together with 2 and 5, are all the Eisenstein primes of absolute value not exceeding 7.
As of September 2019[update], the largest known (real) Eisenstein prime is the ninth largest known prime 10223 × 231172165 + 1, discovered by Péter Szabolcs and PrimeGrid. All larger known primes are Mersenne primes, discovered by GIMPS. Real Eisenstein primes are congruent to 2 mod 3, and all Mersenne primes greater than 3 are congruent to 1 mod 3; thus no Mersenne prime is an Eisenstein prime.