You are correct, the usual chemical suppliers for schools only list marble chips up to 10mm. When you are looking for larger chips for use in a Kipp’s apparatus for generating carbon dioxide or comparing reaction rates, this is not always useful.
Heating calcium carbonate, marble chips
Here are some links to practical activities for heating calcium carbonate, including some using limestone as a source:
- https://www.creative-chemistry.org.uk/documents/N-m06-05.pdf (Link updated February 2019)
Some references suggest using chalk, but not blackboard chalk, which is usually calcium sulphate.
These activities recommend the use of pea-sized pieces of calcium carbonate, in which case, the chemical supplier product would be a suitable size to heat.
Science ASSIST has not trialled this activity and recommends a site-specific risk assessment be conducted. If larger pieces are used, it is recommended that this be trialled prior to being conducted in the classroom.
Possible sources of larger pieces of calcium carbonate marble chips are:
- Stonemasons: those that works with marble. They may have “offcuts” to dispose of and may be willing to allow you to obtain some. Check that they are using natural marble and not reconstituted marble, as the reconstituted version will have bonding components added.
- Garden Suppliers: many of them have marble for decorative landscaping. You will need to check that they are actually marble chips and not quartz, further information below.
- Bunnings: Marble pieces of about 20 to 30mm can be found under the heading of Decorative Pebble Tuscan Path, Snow White Crush, 20kg pack for about $20. (Note: the “Fine Line” code is FL 3460 481 and the barcode identifier is 09328 0910 00886)
- Scientific suppliers: marble chips may generally measure around the 8 to 15mm in size, so the larger-sized pieces may be selected.
Is it quartz or marble?
On a casual glance, these two may look similar, but they behave very differently. See the Geoscience mineral identification flowchart: https://d28rz98at9flks.cloudfront.net/79626/79626.jpg (last accessed 5-2-2016).
Quartz: is one of the two most abundant minerals in the Earth’s crust, its composition is SiO2. 
Marble: is metamorphosed limestone and has a composition of CaCO3.
To identify whether the sample is quartz or marble, here are some simple suggestions.
- Marble (calcite) is crystalline CaCO3, quite isometric and not foliated (or layered). It has a Moh’s hardness of 3, so will not scratch steel or glass—whereas quartz (Moh’s hardness of 7) will.
- A simple acid test with dilute acid (0.5 to 1M HCl or vinegar) produces bubbles of CO2 due to the carbonate in calcite/marble/limestone—silica- or quartz-based rocks do not contain carbonate and therefore do not produce CO2 when dilute acid is added.
Some marble rock specimens from some of the above sources may be too large for your requirements, but marble is relatively soft and easily broken. After conducting a site-specific risk assessment on the crushing of the stones, it can be broken into smaller pieces by wrapping a few stones in a cloth, placing the cloth on a piece of timber that is on a firm surface and hitting firmly with a hammer while wearing appropriate PPE, including safety glasses, until the desired size is obtained.
 Nuffield Foundation. Thermal decomposition of calcium carbonate. 2012 http://www.rsc.org/learn-chemistry/resource/res00000704/thermal-decomposition-of-calcium-carbonate
 Geoscience Australia, 2014. Mineral identification flowchart. Geoscience Australia, Canberra. https://d28rz98at9flks.cloudfront.net/79626/79626.jpg (last accessed 2-5-2016).