The use of jewels and gems in this excellent Shakespearian comedy are primarily to indicate both wealth and regard. In the quotations listed beneath the question, jewels and jewellery are sent by both Olivia and Orsino as tokens of love, esteem and wealth. Note for example quotation number 5, which is said by Olivia to Viola in her disguise as Cesario:
Here, wear this jewel for me, 'tis my picture--
Refuse it not, it hath no tongue to vex you--
And I beseech you come again tomorrow.
In this case, as with the majority of the other quotations, jewellery is given and sent as a symbol of love and regard. Olivia's reference to the jewel she has just given to Cesario, "'tis my picture," is interesting as she recognises that the jewel she bestows shares a similarity with her. This could be symbolic, in terms of pointing towards the wealth that she has through the value in the jewel, but it could also point towards the beauty of Olivia as expressed through the beauty in the jewel. Jewels therefore are used both as symbols of the wealth of those who give the jewels and as a promise of the wealth that the receiver of the jewel will gain through marriage to the giver. They act as personal advertisements of the giver's wealth and beauty. The only other quote referenced above is quotation number 4, where Malvolio imagines his life when he is married to Olivia. Again, here jewels are a status symbol of the wealth that he will enjoy.