Department stores and tourist-oriented shops in Bangkok city open at 10am or 11am and close at about 9pm. 

The Central department store (two branches: one on Thanon Silom, the other on Thanon Ploenchit) is probably the citys best, but Robinsons (on Sukhumvit Soi 19, Thanon Rajdamri and at the Silom/Rama IV junction) is also good. 
The massive Chatuchak Weekend Market is a marvellous shopping experience.

The government souvenir centre
Narayana Phand at 127 Thanon Rajdamri, stocks a huge assortment of reasonably priced handicrafts from all over the country; Silom Village has some classier stuff, but for more unusual crafts, go to the pricey Rasi Sayam (Mon-Sat 9am-5.30pm), a ten-minute hike down Sukhumvit Soi 23, opposite Le Dalat Vietnamese restaurant. Come Thai, second floor of the Amarin Plaza on Thanon Ploenchit (no English sign, but has a carved doorframe), holds an impressive range of unusual handwoven silk and cotton fabrics, much of it made up into traditional-style clothes. 

Shopping in Bangkok

Thai silk is noted for its thickness and sheen; prices start at about B350 per yard for two-ply silk, B500 for four-ply. Jim Thompsons Thai Silk Company at "9 Thanon Suriwong" is a good place to start looking. Inexpensive silk and tailoring shops crowd Silom and Khao San roads, but be wary of places offering ridiculous deals - when you see a dozen garments advertised for a total price of less than $200, you know the quality is poor. If you cant decide, try the reputable Ambassador Fashions at 11 Sukhumvit Soi.

Bangkok has some excellent English-language bookstores : Asia Books has branches on Thanon Sukhumvit between sois 15 and 19 and between sois 4 and 6; in Siam Discovery Centre on Thanon Rama I; in Thaniya Plaza near Patpong. DK (Duang Kamol) Books has branches in MBK shopping centre; at 244-6 Soi 2, Siam Square; and on Thanon Sukhumvit. Also recommended is Tek Heng Books on Thanon Charoen Krung, north of the turn-off for Thanon Silom. For second-hand books, you cant beat Shaman Books, which has two branches on Thanon Khao San, Banglamphu; all books are displayed alphabetically and logged on the computer, but dont expect bargains.

Bangkok is the place to buy cut and uncut stones such as rubies, sapphires and diamonds. 

Recommended outlets include Johnnys Gems - 199 Thanon Fuang Nakhon, Merlin et Delauney - 1 Soi Pradit, off Thanon Suriwong, and Uthai Gems - 28/7 Soi Ruam Rudee, off Thanon Ploenchit. 
Be extremely wary of touts and the shops they recommend: its common to charge a lot money more than what the gem is worth based on its carat weight. 
Get the stone tested on the spot, and then ask for a written guarantee and receipt. 
Unless youre an experienced gem trader, never consider buying gems in bulk to sell them elsewhere: many a gullible traveller has invested thousands of baht on worthless multi-coloured stones.