got this from gamefaqs
I thought there was another way to get it, but you had to be very lucky to get it... if i were you i would just restart
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____ ___ ____ [SQ01-1]
/------\ /_ / ___ ___/ (_)__ _____ / __/__ ___ ___ _____ /------\
\------/ / /_/ _ \/ _ / / _ `/ __/ _\ \/ _ \/ -_) _ `/ __/ \------/
[SQ01] /___/\___/\_,_/_/\_,_/\__/ /___/ .__/\__/\_,_/_/ [SQ01-0]
/_/
In order for the Zodiac Spear to be available in the chest later in the game a
few treasure chests must NOT be opened as you journey through the main story of
the game.
[ ] Do not open the treasure chest directly across from Old Dalan's front door
in the South Sprawl part of Lowtown. [SQ01-1]
Pic: http://tinyurl.com/3bshnu
[ ] Do not open either of the two chests in the Cellars area of the Royal
Palace of Rabanastre. This is the area where the red Seeq distracts the
guard while Vaan sneaks by. [SQ01-2]
Pic: http://tinyurl.com/3ds66o
[ ] Do not open any of the chests in The Confiscatory of the Nalbina Dungeons.
This is the room where the party gets their weapons back after being
captured. [SQ01-3]
Pic: http://tinyurl.com/2mag9l
[ ] Do not open any of the 16 chest on the Phon Coast that are lined up along
the beach of the Vaddu Strand area. All of them contain gil, but you want
that spear more than a one-time helping of gil right? [SQ01-4]
Pic: http://tinyurl.com/2uezoo
The Zodiac spear can be found inside of a treasure chest in the Necrohol of
Nabudis/Cloister of the Highborn if you do not open any of the above chests.
See Necrohol of Nabudis/Nabreus Deadlands section under this section for more
info. The spear has a whopping AP of 150 and is the most powerful weapon in
the game. Enjoy!
IMPORTANT: Be sure to NOT equip the Diamond Armlet when attempting to collect
the spear from the chest or your party will receive a Dark Matter instead!