Statements: Some pearls are stones. Some stones are diamonds. No diamond is a gem. Conclusions:  Some gems are pearls. Some gems are diamonds. No gem is a diamond. No gem is a pearl.

III is the converse of the third premise and so it holds.

Some pearls are stones. Some stones are diamonds.

Since both the premises are particular, no definite conclusion follows.

Some stones are diamonds. No diamond is a gem.

Since one premise is particular and the other negative, the conclusion must be particular negative and should not contain the middle term. So, it follows that 'Some stones are not gems'.

However, I and IV involve the extreme terms of the three premises and form a complementary pair, Thus, either I or IV follows.