Before everyone forgets, the original scapular was an apron that resembled a poncho which was worn over one’s outer tunic. We still see them in some religious orders who wear the older habits, such as Dominicans, Carmelites, Benedictines, and some Franciscan congregations.
The Dominicans’ scapular is not wool, nor brown. It’s white and usually made of some wash and wear fabric, as are most religous habits today.
The little scapular came out of convenience for those who could not wear the full-length scapular that friars, monks, and nuns wore.
What material it’s made of is probably more a matter of how easy it is to care for and what is available in your location. I can’t imagine wool being very available in the Rain Forest or in desserts.
After Vatican II, Secular Franciscans replaced the small brown Franciscan scapular with a metalic one. It looks like T with two hands crossing it. It can be worn over or under one’s clothing.
As to investing one in the scapular, that is a ritual for those who are becoming members of a religous order or associates. But that does not mean that others can’t wear a scapular. Of course there are scapulars that are specific for members of certain religious families.
For example, Secular Franciscans are members of a religous order, even though they are secular men and women. Their scapular is very specific. But they probably will not be offended if a non Franciscan wears it. On the other hand, the friars do not wear a scapular. Their Constitution is very clear on what their habit should be.
The scapular question is simple. If you want to wear it, do so, with the proper respect and the proper spiritual attitude. Make it out of something that you can care for easily.
It’s just the tiniest bit bulkier than a regular Brown Scapular, but is 5 scapulars in 1, for even more protection and grace. It’s like a 5-for-1 deal 
