B737-300 FLT CREW HAS PRESSURIZATION MALFUNCTION; DECLARES EMER; EXECUTES DSCNT AND DIVERTS FOR LNDG.

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INITIALLY CRUISING AT FL260. ENCOUNTERED CONTINUOUS MODERATE CHOP. WE RECEIVED A PIREP FROM DISPATCH RPTING THAT FL300 WAS A BETTER RIDE. AT THIS TIME I WAS THE PNF AND THE CAPT WAS THE PF. UPON CLRNC TO FL300; I RESET THE AUTO CTLR FROM FL260 TO FL300 AND RESET THE STANDBY CTLR TO THE PRESCRIBED CABIN ALT. AFTER APPROX 5-10 MINS AT FL300; THE CAPT CALLED THE A FLT ATTENDANT TO COORDINATE THE SECURITY PROCS FOR USING THE RESTROOM. THE FLT ATTENDANT RANG US AND THE CAPT VERIFIED THAT THE CABIN WAS SECURED AND LEFT THE COCKPIT AND THE A FLT ATTENDANT CAME INTO THE COCKPIT. PRIOR TO LEAVING HIS SEAT; THE CAPT POSITIVELY XFERRED AIRPLANE CTL TO ME AND I MAINTAINED ATC DUTIES. I HAD THE OXYGEN MASK ON WHEN THE CAPT LEFT HIS SEAT. THE FLT ATTENDANT SECURED THE COCKPIT DOOR. VERY SHORTLY AFTER THE DOOR WAS SECURED I HEARD THE INTERMITTENT CABIN ALT WARNING. I LOOKED UP TO THE CABIN ALT INDICATOR AND IT INDICATED OVER 10000 FT AND THE RATE INDICATOR WAS CLBING AT 700+ FPM. I SWITCHED MY RADIO PANEL TO MASK AND LIFTED THE SPEAKER VOLUME UP. I LIFTED MY MASK MOMENTARILY AND EMPHATICALLY TOLD THE A FLT ATTENDANT TO GET THE CAPT BACK INTO THE COCKPIT. AT THAT POINT I GRABBED THE QRC AND CHANGED THE ALT SELECTOR TO A LOWER ALT. I PRESSED LEVEL CHANGE AND DID NOT NOTICE ANYTHING HAPPENING. I THEN ADJUSTED THE AIRSPD INDICATOR TO AIRSPD AND SPED IT UP TO 320 KTS. I STILL DID NOT NOTICE MUCH OF A DSCNT SO I DISCONNECTED THE AUTOPLT AND BEGAN A FAST DSCNT AT APPROX 4000 FPM. I ALSO DEPLOYED THE SPD BRAKES. I THEN NOTIFIED ATC THAT WE WERE DSNDING AND RECEIVED NO RESPONSE. I MOVE THE SPEAKER VOLUME TO HIGHER AND STATED 'ACR X IS IN AN EMER DSCNT; NEED 10000 FT.' AT THIS POINT I LOOKED UP ABOVE MY HEAD AND THE OUTFLOW VALVE INDICATOR WAS IN THE MIDDLE POS (I THOUGHT TO MYSELF IT IS NORMALLY NEAR THE FULL CLOSED POS AT ALT). WHILE APPROX THROUGH FL270; THE CAPT SAT BACK DOWN AND GRABBED THE QRC FROM MY HAND. SINCE I WAS HAND FLYING THE PLANE; I DID NOT READ THROUGH THE QRC. THE CAPT THEN SET 7700 IN THE XPONDER. I THEN OFFICIALLY DECLARED AN EMER AND INDICATED TO ATC THAT WE HAD A PRESSURIZATION PROB. THE CAPT RAN THE EMER DSCNT QRC AND THEN THE EMER DSCNT CHKLIST IN THE FM WHILE I FLEW THE AIRPLANE DOWN TO LOWER ALT. DURING THE ENTIRE TIME THE CABIN ALT HORN WAS ON. ONCE LEVEL AT 10000 FT (280 KTS) THE CAPT STARTED COORDINATING WITH DISPATCH; MAINT AND THE FLT ATTENDANTS IN THE CABIN. THE HORN WAS STILL GOING OFF AND I REQUESTED 9000 FT AND SLOWED THE AIRPLANE. APPROX 4-5 MINS AFTER BEING LEVEL AT 9000 FT THE ALT HORN FINALLY SILENCED. AFTER SOME VECTORS AND A 'PPOS' HOLD; WE DECIDED TO LAND AT ZZZ. THE CAPT HAD PULLED UP THE ZZZ ATIS AND I PUT THE AIRPLANE BACK ON AUTOPLT. WHILE THE CAPT CONTINUED COORDINATING WITH DISPATCH AND MAINT; I SET UP THE FMC FOR ILS APCH AND DETERMINED APCH SPDS. CAPT WAS DONE COORDINATING WITH COMPANY AND THE CABIN; HE THEN TOOK OVER THE PF DUTIES AND I ASSUMED THE PNF DUTIES. THE CAPT THEN BRIEFED THE APCH AND I COMPLETED THE APCH/DSCNT CHKLIST. AFTER SOME VECTORING WE COMPLETED AN UNEVENTFUL LNDG AT ZZZ. DUE TO BEING IN THE COCKPIT WITHOUT THE CAPT I DECIDED THE BEST COURSE OF ACTION WAS TO INITIATE THE EMER DSCNT WITHOUT READING THE QRC. CAPT DEBRIEFED THAT THE MAX CABIN ALT HE SAW WAS 12000 FT AND THE MASKS WERE NOT DEPLOYED IN THE CABIN.