Luster is also measurable-- e.g. hanadama pearls are graded on their teri-value.
Luster is dependent not only on the thickness of the nacre, also on how well the crystals of aragonite are arranged.
So while thick nacre is not a guarantee of high luster, a pearl won't have high luster without also having thick nacre. For example-- freshwater pearls without a nucleus are all nacre, but there are many dull FWP.
Also, pearls need to be properly cared for or they can be damaged. Exposing them to perfumes, hair spray, cosmetics etc. will damage the luster.
Storing them in plastic bags prevents their having access to moisture in the air which is good for them. Opals also need moisture.
Edit: I'm clarifying due to confusion this generated: Moisture is good for pearls, opals etc. Air contains moisture, so it's good for pearls to have exposure to air. Plastic bags keep moisture out and so are not good for long term storage of pearls.