Here is how I made a realistic fossil matrix for my children to excavate. Use this as part of a fossil excavation activity. The first time you do this I highly recommend making a small trial matrix first using a few rocks instead of fossils - something about the size of your hand, just so you know how much of your natural materials to use, how quickly to you expect it to dry.

​When you are done you'll have a realistic looking "rock" with fossils embedded. It should be hard enough that use of a hammer and pick might be warranted, but not so hard that it frustrates the kids

What you'll need

• Plaster of Paris - an 8 lb bucket of powder is a good size to make a large rock
• Dry natural particles - dirt, very small gravel, or barbecue ash. I like barbecue ash because it gives the resulting rock a nice natural color and makes the rock a little harder.
• Fossils - about 10 to 20 fossils, depending on size. I like to put at least on crown jewel such as a large spinosaurus tooth, a mosasaurus tooth or a megalodon tooth. Something large. Do not use replicas.
• A foil roasting pan - you can get them at most grocery stores.
• A bucket to mix the plaster - something you don't mind getting dirty - not on you plan to use for cleaning the kitchen floor
• Something to mix the plaster - sturdy stir stick, garden shovel ... not your nice wooden kitchen spoons. 

Partially excavated fossil matrix.

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What to do

• Set out your fossils that you plan on embedding into the matrix so you have them ready. Once poured, you don't have too long to get them pressed into your 'rock'
• Fill your foil roasting pan with about an inch of dirt. This helps prevent the matrix from taking the shape of the bottom of the roasting pan.
• Empty your plaster of paris powder into you bucket.
• Add your ash and dirt to the powder in about a 2:1 ratio. Two parts plaster powder to one part natural material. This natural material will give your matrix a more natural color and texture than just plaster.
• Stir the dry ingredients - the don't need to be mixed perfectly even.
• Begin adding water. Add it slowly and mix constantly as you add the water. The it should be thick but pourable - not watery. DO not exceed the amount of water specified in the instructions that came with your plaster of Paris. Once mixed evently- it should be a light grey/brown depending on what natural material you mixed in.
• Pour the mixture into the pan, over the dirty. Move the bucket around a bit to fill space but avoid the edges of the pan - you want this to be relatively rock shaped and not pan shaped. You shouldn't have to spread or press the mixture around the pan but if you want to poke and scrape at it so it is not so smooth - that's fine.
• Add the fossils. Start pressing the fossils into the plaster mixture. Depending on the patience/tolerance of your children you can either just press them in a little so that they are easily seen (better for younger ones) or  embed them deeper and push some of the master mixture over then (for older or more patient fossil hunters). I like to keep at least part of the fossil showing, even if it is just a tiny tip of something, but that's your call.
• Let the matrix dry. Drying time depends on how much water and how thick the rock is. When dry it should feel hard, very rock like.
• Once completely dry, lift it out of the pan and brush away as much dirty as you can from the bottom. Be gentle - it may feel like a rock, but it easily breaks and chips.

You now have a fossil matrix.

Dry mixture of plaster of Paris, barbecue ash and dirt.

Mixture after adding water. Thick but still pourable.

Matrix with fossils embedded - still wet. This would be considered 'easy to find' placements of fossils.