In my research, I believe that only two glass firms ever produced intaglio.
These were Dugan Diamond and Northwood.  Northwood did not mark all his
intaglio pieces with the characteristic N .  Some are and some are not marked.

A number of Northwood pieces did however stand out from the Dugan pieces
in that quite a few were noticeably thicker and heavier and had fired gold edges. 
I always felt the Dugan Diamond intaglio was thinner and that their art work is
superior to H. Northwood's. 

"Dugan / Diamond - The Story of Indiana, Pennsylvania Glass"  by
Heacock, Measell,  & Wiggins states the Dugan Glass Company began to market
its Intaglio line in 1905.  It is described thusly:

"... pressed crystal glass which has a pattern (often a fruit motif) on the outside 
surface of such articles as bowls, plates and stemmed comports. The pattern
 is not raised from the surface of the glass as is typical in pattern glass; it is
 below the surface of the glass, i.e. in hollow relief"

It was not uncommon to find Intaglio with opalescent treatment.  To be quite
fair,  I would not call such pieces opalescent, but rather intaglio with opalescent
edges or treatment.  

FROM: James Measell,  Fenton Art Glass,   Jan 18, 2000:

"My feeling is that Northwood's and Dugan's Intaglio lines did not last very
long (probably 1906-1909 and 1905-1909, respectively) because these firms
became involved heavily with iridescent ware in 1908. The only known
Northwood catalog (fall 1906) contains 1 page of Intaglio ware showing 11
items. The 1907 Dugan catalog contains about 4 pages on Intaglio. I spent
about six years looking into Northwood and Dugan, and virtually everything I
found relating to Intaglio is in the two books. Despite what is in the
McKinley book, I don't think La Belle was responsible for any goofus glass.
There is much to be learned about Fostoria in this area.

I found many Butler Bothers and G. Sommers and Co. catalogs, portions of
which are used in the Northwood and Dugan-Diamond books to show Intaglio. I
also read all the glass tableware industry trade journals looking for ads. I
talked to all the Northwood and Dugan descendents I could find and spent
much time at local libraries and historical societies where some records are
stored."