Why did Mme. Forestier have a fake necklace in "The Necklace"? Why would Mme. Forestier want to own an imitation diamond necklace when she must have...

Madame
Forester did not tell Mathilde the necklace was a fake because she wanted her to feel good about
herself, thinking she was wearing a real diamond necklace.

In
the plot revolves around Mathilde, a poor woman who borrows a necklace from her friend and
then loses it and goes into debt trying to replace it.  Mathildes husband gets an invitation to
the ball, but she worries that she does not have good enough clothes.  Her husband tells her to
borrow a jewel from her friend Madame Forester, because she knows her well enough to do that
(p. 3).

This implies that Mathilde does not really know Madame Forester
well.  She only knows her well enough.  It makes sense that they would not be close friends,
because they belong to different classes and will run in different social circles.  As a result,
Madame Forester probably does not trust Mathilde enough to lend her a real diamond, but she
wants her to feel good about herself so she lends her the fake one.

Notice
first that Madame Forester does not pick the necklace out.  She offers for Mathilde to take a
look, and she does not like what she sees.  Is it possible that Madame Forester has a lot of
jewels of good workmanship, but not really worth much?  Chances are she has other jewels kept in
a safe place, rather than a box, and those are the real ones.  That is why there was nothing
valuable in the box Mathilde looked through.  When she asks to borrow it, she does not expect
Madame Forester to say yes.

Then she asked, hesitating,
filled with anxious doubt:

"Will you lend me this, only
this?"

"Why, yes, certainly." (p. 4)

There is no hesitation at all.  In fact, Madame Forester seems
surprised Mathilde would doubt her.  She might assume that Mathilde knows that the necklace is
fake.  Her response when Mathilde gives her the real diamond seems to support this
interpretation.  She is deeply moved when she learns that Mathilde worked for so long to pay
off the necklace.

"Oh, my poor Mathilde! Why, my
necklace was paste! It was worth at most only five hundred francs!" (p. 7)

The fault does not lie with Madame Forester.  Mathilde is the one
who lost the necklace and did not tell her friend.  She was the deceptive one.  Madame
Forester did not tell her the necklace was fake, but she did not tell her it was real either. 
She likely assumed Mathilde could tell the difference, or did not want to burst her
bubble.

Your answers are plausible, but one of
the things that bothers me is this: Mme. Forestier either had to assume Mathilde knew or did not
know the necklace was a fake. If Mathilde knew it was a fake, would she want to wear it to an
important ball where knowledgeable men and women would see it was a fake? If she thought it was
real, shouldn't her friend have warned her that it was not, so that the poor, naive young woman
wouldn't spend the whole evening dancinig with men who were well aware it was a fake and would
assume she was trying to deceive people?

My other question had to do with why
a rich woman should collect what was actually junk jewelry? If she acquired it, she must have
worn it herself. If so, wasn't she aware that many people would feel contempt for her because
they would recognize it as junk?