How do I get my tornado to only pick up parts based on their material?

Hi. I'm making a game that involves a tornado. I've been trying to implement different tornado strengths, by making each classification of tornado be able to pick up only certain types of materials. This is the script I have so far with nothing about that, and that works pretty well:

local hole = script.Parent -- Hole is kind of a code name for the tornado part (types easily).
local childList = {} -- Childlist is a list of debris that the tornado picks up.

local massConstant = 5.8

local mass = 32000 * massConstant

function checkObject(obj) -- Obj is the child model.
    if (obj ~= hole) and (obj.className == "Part") then
        if (obj.Anchored == false) then -- Will only make it debris if the part is not anchored.
            table.insert(childList, 1, obj) -- Adds part to child list, making it debris.
        end
    elseif (obj.className == "Model") or (obj.className == "Hat") or (obj.className == "Tool") or (obj == workspace) then -- Just a precautionary measure.
        local child = obj:GetChildren()
        for x = 1, #child do
            checkObject(child[x])
        end
        obj.ChildAdded:connect(checkObject)
    end
end

checkObject(workspace)

print("Tornado Damage Script Loaded.")

local n = 0
while true do
    if n < #childList then
        n = n + 1
        if n % 800 == 0 then
            wait()
        end
    else
        n = 1
        wait()
    end

    local child = childList[n]
    if (child ~= hole) and (child.className == "Part") and (child.Anchored == false) then
        local relPos = hole.Position - child.Position
        local motivator = child:FindFirstChild("BlackHole Influence") -- Motivator is what causes the part to go around the tornado.
        if relPos.magnitude * 240 * massConstant < mass then
            child:BreakJoints()
            if (relPos.magnitude * 320 * massConstant < mass) and (child.Size.z + hole.Size.x >  relPos.magnitude * 2 - 4) then
                mass = mass + child:GetMass()
                table.remove(childList, n)
                n = n - 1
                motivator:Remove() -- After a certain point, the debris will be spat out of the tornado, creating realistic tornado damage.
            else
                child.CanCollide = true
                if motivator == nil then
                    motivator = Instance.new("BodyPosition")
                    motivator.Parent = child
                    motivator.Name = "BlackHole Influence"
                end
                motivator.position = hole.Position
                motivator.maxForce = Vector3.new(1, 1, 1) * mass * child:GetMass() / (relPos.magnitude * massConstant)
            end
        elseif motivator ~= nil then
            motivator:Remove()
        end
    end
end

What I had in mind at first was to insert an "and" statement into the "if/then" statement at line 10, as so:

        if (obj.Anchored == false) and (obj.Material == "Pebble") then -- Will only make it debris if the part is not anchored.
            table.insert(childList, 1, obj) -- Adds part to child list, making it debris.
        end

"Pebble" is just an example for a material. I tried this and suddenly the tornado did absolutely no damage. What am I doing wrong?