I need a few semi-precious stones/rocks as a present - not the polished ones, but those raw ones which look like a rock and are cut, inside it´s the semi-precious thing (sorry, can´t explain better).

Found one shop at MBK, but the three stones they had were not beautiful. Not looking for something expensive, small stones would be perfectly ok.

Thanks for any ideas!

Best place to go to is the Jewellery Trade Centre on Silom Road (river end, right next to the Holiday inn). Lots of jewellery shops in the Galleria area, and many gem stones available. If you look (and ask) around you will also find some that sell rough stones, e.g. Ruby, sapphire, aquamarine etc. You may think that the shops look a bit up market, but I believe prices are reasonable, and it's where all the traders go. If not some of the many other gemstone shops in the area may have some rough stones.

If you want synthetics and imitation stones go to the Wat Ko area in China town (Wat Ko is now named Wat Samphanthawong on maps). There are a number of Indian owned shops on Soi Wanit 1 which may have some rough stones, although I think they mainly trade in synthetics and imitation. I picked up a half boule of synthetic ruby and also a chunk of ruby red cubic zirconium last weekend.

Try PIS building, Mahesak Rd, between Soi Pramot 3 and Silom, and the surrounding area up to Charoeng Krung Rd. There are many stone wholesalers, not sure about the minimum quantities or which of the shops also do retail, but that's the area for stones in Bangkok.