Tiffany in 1858 bought the unused cable in the hold of the S.S. Niagara from Cyrus W. Field, cut it into pieces and sold it as souvenirs.

Below are images of this cable, and the facsimile of the letter showing the purchase (this came with the cable piece)

The piece is 3" long, by 1.2" diameter.
The cable is bound at each end with a piece of embossed brass.
The upper brass cap carries the inscription:
"Telegraph Construction & Maintenance Company Limited - Contractors London"
The lower brass cap carries the inscription:
"Societe Du Cable Transatlantique Francais Limited
- Brest To New York 1869 - Deep Sea Cable".

Used to raise 330,000 GBP Capital

(Note: above image converted to black and white)

Stamp detail:

Pamphlet for those touring the Great eastern