I'll assume this definition of specifiable:
A real number is specifiable if it can be written in a finite sequence of symbols from a finite alphabet.
If that's the definition you're thinking of, then the answer to your question is yes, since the set of finite sequences of symbols from a finite alphabet is countable.
That is not necessarily true for an infinite sequence. See GeorgCantor's diagonalization argument (CantorsProof).
Note that the specifiable real numbers are not a subset of the rational numbers. For example, the definition
x = square root (2)
uniquely specifies an irrational number using a finite sequence of symbols from a finite alphabet.
See AreTheSpecifiableRealsWellDefined, though - it's not clear the above is a coherent definition. In fact, it depends on whether the "language" in which the numbers are specified is well-defined - i.e. whether each sequence of symbols maps unambiguously to a real number.
There are numerous interesting (and countable) subsets of the reals (which of course are not countable).
Among them:
- The integers
- The rationals
- The "constructable" numbers -- expressible as a finite sequence of addition, subtraction, multiplication, division, and Pythagorean sum (sqrt (a*2 + b*2)) -- essentially, those numbers whose magnitude can be computed with compass and straightedge (see SquaringTheCircle)
- The "simple algebraic" numbers -- expressible as a finite sequence of addition, subtraction, multiplication, division, and integer root extraction (square root, cube root, etc.). All roots of polynomials of degree 4 or lower are in this category; higher-order polynomials may have roots which are not so expressible.
- The algebraic numbers.