One possible answer is that there are two differences between the characteristics of the other materials and those of the oil and spices. Firstly, while the other materials were important, they required no effort in producing, while the oil and spices had to be manufactured and maintained. Those people that didn't have the precious stones to donate to the building of the Mishkan still had the opportunity to contribute with their efforts instead! Secondly, both the oil and the spices are of the most 'giving' materials used in the Mishkan; The oil was used to light the Menorah, which gives off light to everything around it, and the spices give off a beautiful smell to its surroundings. The message it clear...The most beautiful and giving things in life are those that require our active effort. Spices smell and oil illuminates BECAUSE someone took the time and effort to make them! The same can be said today...Being a good person and a good Jew is beautiful and rewarding to ourselves and to others, but only BECAUSE we take the time and effort to understand and cultivate it!