If this is a Whirlpool model top loader, the model number will be located on a nameplate along the wash tub rim under the lid.
There is no screen or strainer basket in the drain pumps on these washers. The pump is strictly mechanical and has only one moving part (the impeller) and two hoses attached. The following link explains how you may access the pump and remove (if necessary) to inspect:
http://www.fixya.com/support/r3794382-replacing_water_pump_whirlpool_top
Now, before you dismantle the washer to look for your lost diamond, did you run the washer after the diamond was dropped? If so, it may have already been run through the washer and out the drain line. Or, it could be lodged in the drain line somewhere. You can use a wet/dry shop vac to pull a vacuum on the line and check the contents of the vacuum for your lost stone. If you do not own a shop vac, you will need to move the washer to a location where you can place the drain line on the floor to allow simple gravity to allow the water to run out.
On some of these top loader models there is a screen in the wash tub under the spin basket. You can tell by removing the inlet hose to the drain pump and feeling up inside the wash tub. If there is a mesh screen, you will need to remove the agitator and spin basket to access the inside of the wash tub to inspect. The process is not as difficult as it sounds. Remove the exterior washer case as outlined in the link I provided previously. Then, follow these steps:
Start by removing the agitator:
1. Remove the agitator center cap by either pulling straight up or gently placing a flat blade screwdriver into a slot located in the cap.2. If the agitator is equipped with an inner cap with a handle, remove this as well by pulling straight up.3. Remove the center bolt of the agitator. You will have either a 7/16” hex bolt or a recessed square plastic nut. If you have the square recessed type, use a 3/8” to ½” ratchet adapter to remove.4. Remove the agitator by grasping the bottom flange and carefully pulling straight up. Try to keep all the parts assembled while doing this, so do not pull too hard. This will make this repair much easier.
Then remove the tub balance ring (this is the plastic ring at the top of the wash tub:
1. With a large flat blade screwdriver, locate the tabs holding the ring in place and gently pry each one up to release the ring.
2. Work your way around the perimeter of the tub, prying gently.
3. Once the tabs are all released, make there are no dispenser hoses attached (on some models) and lift the ring out of the way.
Remove the spin basket (this is the inner wash basket):
1. There should be a a center bolt (called a "spanner nut"). The nut usually requires a spanner wrench to loosen, but you may get by with a large pipe wrench or channel locks.
2. Once the spanner nut is removed, the spin basket should lift out of the wash tub.
Under the spin basket there should be a small opening above the drain pump that has a mesh screen over it (on some models). If you lucky, your diamond may be trapped there.
I would really need a complete MODEL NUMBER to be sure how your washer is configured. Please post back if you have questions. I hope this helps you.