Returning to the subject...
Can this pearl be a rare considering its shape and good luster
I've been living with the impression that relatively small Tridacna pearls are usually symmetrical baroques like this one. Of course, not all might have that perfect, classic
'sugarloaf'shape like this one does - it's got to count for something.
Not sure what counts as good / bad lustre for these pearls. I just have not seen that many, and the few I know looked like porcelain (i.e. with pretty good lustre). Rare and unusual as all Tridacna pearls are, I'm wondering whether anyone has come up with a story of what constitutes 'good quality' for them. If one is of a shape and size suitable for wear (like this one), I'm sure it gets extra points. Just from my point of view, even texture and spotless color makes up 'quality'. I am not sure if 'defects' on the underside of a button pearl (like this one) would even be a reason for discount.... no more then they are for natural nacreous pearls, perhaps.
Another question may be whether Tridacna pearls may be compared with the other, better known non-nacreous pearls (Melo and Conch) which loose points for lack of color. Although this comparison makes a bit of sense, it also seems unfair since Tridacna pearls are always white. Your call to put this bit straight
Basically, I believe that at this point no one came out of the woods with a bag (i.e. large stock) of Tridacna pearls to promote the type. And no one came up with a quality standard as yet - even Gems & Gemoloy has nothing on them yet.
The above is just my own 'idea' of what could go into such a quality standard. But I do not have the bag of pearls... so that doesn't really count.
All in all, I find the subtle look of these pearls (i.e. definitely not the showy kind of jewelry, are they) extremely attractive: the 'objects' have instant appeal and invite explaining. Not everyone wants their jewelry to be a 'conversation' piece, but that's another story.