1. After learning Venom Knife, talk to Esmille(the NPC on the right side). She will teach you the skill only after you've completed a task. Kiltin asks you to retrieve a jewel from either Sphinx B2 or Pyramids 4F.
2. In Sphinx B2, there are two places where you can obtain the gem. You only have to get one jewel from either of these locations (If the first doesn't work go the the second one, and vice-versa). The first coffin site can be found at 1 o'clock (216,268), approximately two screens west of the entrance to Sphinx B3, where there will be a coffin on a raised platform. Walk up to the platform to activate the dialogue and you will be prompted to make a choice. Depending on what you choose you will be poisoned, blinded, or cursed. Just be persistent and continue selecting and eventually you will receive a Sapphire.
3. The second coffin site is at around 9 o'clock (18, 163), where you will find a statue with an ornament. It's booby-trapped and will put you to sleep with gas. Again, keep trying and you will receive a Ruby.
4. In Pyramids 4F, the gem is located at middle of the left side of the enclosed water area. Keep walking around it to get an Aquamarine.
5. Return to Esmille with the gem to receive the Sonic Acceleration skill.