Help Amulet Stackable l2jacis

Hi everyone, I need to make the amulets stackable, as well as the spellbooks, but I need to control their consumption. I have modified the item.xml like this for example:
 

	<item id="8885" type="EtcItem" name="Amulet: Magnus' Chant">
		<set name="material" val="PAPER" />
		<set name="weight" val="120" />
		<set name="price" val="120" />
		<set name="is_stackable" val="true" />
	</item>

But I can see that within the spellbooks.xml it handles, I believe its consumption since I see the following for the same amulet:
 

<book skillId="1413" itemId="8885"/><!-- Amulet: Magnus' Chant -->

My question is the following, how do I make sure that when these are stacked, they are not all consumed in a single use.