Grammatically Correct: The writer's essential guide to punctuation ...

The Em Dash (-)
The em dash serves the following functions:
• Marking off a descriptive element or digression
• Marking an abrupt break in structure or turn in content
• Indicating interrupted or scattered speech
• Setting off the source of a quotation
MARKING OFF A DESCRIPTIVE
ELEMENT OR DIGRESSION
A pair of dashes can be employed in two ways: It acts to draw
particular attention to elements that you wish to emphasize, and it
lets you veer off in a different direction temporarily and then get
back on the original track.
In the flrst role, dashes may be used to set off such elements as
identifying information, descriptions, examples and lists. In this role,
they function much as commas do (see the discussion of parenthetical
text under "Comma" on page 71). Consider using them instead
of commas if you want to emphasize an element; if the element is
not integral to the sentence and you want to distance it somewhat;
or if the element consists of a series of subelements that already
contains commas, and surrounding it by more commas might make
the punctuation difficult to follow. More than one of these reasons,
of course, may apply in a given situation.
154
Even Georgette-a girl who knew which side her bread was buttered
on-had her doubts.
The board members-with the conspicuous absence of the president­
met to discuss the missing funds.