These carnelians do present a very nice coloured red rim. If you cut them You´ll see, that in the center this colour is replaced by bluish, yellowish or greyish tones.
Agates of this type do occur all over the world ( e.g. in Oregon, or in Brasil - here they are called “the dry red ones”) in iron-bearing soil that is giving them in a long during process secundary colorization. Originally the colorization is only a weak yellowish tone. By giving them a heat-treatment they are changing the colour to a vivid red. This could be happen by natural processes - such as drying and heating by the sun - or man made by heating the agates up to a temperature of no more than 350°Celsius.
The madegassian carnelians shown here are all in original condition - only cut and polished - not heated. They are showing the colour-concentration in the outer rim. Those are gravel-agates - at the moment no more information about the locality can be given. Sometimes they are showing a very nice structured husk.